2 - The Silence of the Past

The Nanjing Belt

The most puzzling archaeological mystery of ancient China came to light in the true tradition of buried treasure when a workman's spade broke through the roof of a long-forgotten tomb.

It was 1 December 1952. The Jingyi Middle School of Yixing City in the Jiang-su Province of China was building a sports field. The first task was to level the ground, for the school authorities had chosen a patch of land dominated by an oblong hillock. Another feature of the site, four curiously shaped mounds, complicated the work. That day, a labourer's shout brought everything to a halt. His spade had penetrated a thin layer of earth and rubble and a hole had appeared, releasing a rush of musty air. Peering into the darkness, the work gang could dimly make out a chamber stacked with dusty objects.

They called the police, who climbed down through the opening and soon announced that the workers had made a major archaeological find. The place was obviously a tomb. After taking into safekeeping a motley collection of grave goods, including five pieces of porcelain, eleven of pottery, some scraps of gold, two pottery stands and 'four gold articles', the police sealed the chamber and the Huadong Historical Relics Working Team was summoned to conduct a full-scale excavation.

The dig revealed that there were in fact two tombs, both built in an unusual style. Each had chambers with an arched roof constructed in wedge-shaped bricks, topped with a square slab. More bricks, laid in a herringbone pattern, covered the floor. The roof was adorned with carvings: circles, tigers or the faces of animals. Conveniently, the 'Number 1 tomb' contained an inscription which enabled the archaeologists to date their find precisely. On one side it read: '20th September of the seventh year of Yuankang the late general Zhou ...'; on the other, the tomb builders had left their official titles and signatures: 'Yicao Zhu Xuan, jianggongli Yang Chun, workman Young Pu made.' This, then, was the burial-place of a nobleman called Zhou Chu.

Zhou could be traced in the historical records. A renowned military man and scholar, he had lived during the Jin Dynasty (A.D. 265-420), and had died fighting the Tibetans in 297. There could be no doubt whatsoever about the dates, and this made one discovery all the more astonishing. Encircling what had once been the waist of a rotted skeleton found in the 'Number 1 tomb' were about twenty pieces of metal, obviously the remains of a belt. 'The factor worth noting,' wrote archaeologist Luo Zong-chen with what proved to be breathtaking understatement, 'is the chemical composition of these ornaments.'

For analysis of one fragment by the Chemistry Department of nearby Nanjing University revealed that it was composed almost entirely of aluminium. Now, although aluminium is widely used in modern life, it was not isolated in the West until the early years of the nineteenth century, and a generation later was still so rare that it was a showpiece of the 1855 Paris Exposition. The production of aluminium requires something thought to have been quite unknown in ancient China: electricity. The discovery of the belt therefore raised a question which fascinated archaeologists, metallurgists and chemists both inside China and far beyond its boundaries: did the Chinese beat European scientists to the isolation of aluminium by a cool 1,500 years?

While it would be going too far to say that archaeologists have become used to pondering such problems - their working lives are usually devoted to the painstaking accumulation of more prosaic evidence of the daily lives of ancient peoples - a few objects like the Nanjing belt have presented them with an irresistible and potentially unsettling challenge. Did the scientists, artists and builders of the past know secrets that their successors have taken centuries to rediscover? Should our ideas about the level of technology achieved by the ancients be drastically revised?

How then did the experts set about finding the answer to the puzzle of the Nanjing belt? As so often happens, they first fell to arguing. In China itself the pages of the academic journals were full of the controversy. In the magazine Koagu, one expert, Shen Shi-ying of the North Eastern Engineering College, reported that he had carried out several methods of analysis on a small broken piece of the belt which he had obtained from the Nanjing Museum. 'But,' he announced, 'the results of these various analyses all pointed to these alloys being silver-based rather than aluminium-based.'

Another piece gave similar results, but yet another fragment, originally sent to a different analyst, really did seem to contain aluminium. Yet Shen Shi-ying remained sceptical and concluded: 'It is impossible to tell from its structure whether it was made in ancient times. At the same time, it was unlike the product of a 1960s factory.' He suggested that the aluminium might have been made at the beginning of this century, but added cautiously, 'This is only a supposition, and to know definitely, all round research in depth is called for.'

Stung by Shen's reservations, and particularly by his suggestion that the piece of metal which analysis had proved to be aluminium had been introduced into the tomb at a much later date, perhaps by grave robbers (who had undoubtedly broken into the tomb at some time in the past), one of the original excavators, Luo Zong-chen, published a riposte.

The belt pieces, he wrote, were certainly of the Jin period, for most of them 'were underneath the accumulated earth, showing that they had never been disturbed'. Luo also attacked Shen's assertion that most of the fragments found had turned out to be silver. Four pieces, he conceded, had indeed been shown to be silver, but four others were made of aluminium.

The Cultural Revolution of the 1960s which so disrupted Chinese academic life, brought the controversy to an abrupt end with nothing resolved, but by then the story was out. One of the many experts in the West who learned of the Nanjing belt was Dr Joseph Needham of Cambridge University, author of the monumental Science and Civilization in China, and perhaps the greatest authority of all. He was intrigued and did not entirely dismiss the idea that the ancient Chinese had somehow found a way to isolate aluminium. 'For the present it would be unwise to rule out the possibility,' he wrote in 1974.

One group of Western scientists, however, did not stop at simply expressing interest in the Nanjing belt. In 1980, inspired by Joseph Needham, Dr Anthony R. Butler and his colleagues, Dr Christopher Glidewell and Sharee E. Pritchard, of the Chemistry Department of the University of St Andrews decided to continue the search for the truth, begun a quarter of a century earlier in China. In 1986 their report was published, and was eagerly consulted by scientists and lay people whose curiosity had been whetted by the Chinese controversy. Its title, 'Aluminium Objects from a Jin Dynasty Tomb - Can They Be Authentic?' held out the promise that the three investigators had found an answer to the mystery.

They began by acknowledging that modern research into Chinese science and technology has revealed many previously unsuspected scientific and technological achievements, some astonishingly advanced. 'Consequently we believe that no report of a medieval Chinese chemical achievement, however remarkable, should be rejected without adequate modern re-assessment.' Even so, they judged that the production of aluminium in the Jin Dynasty, an age without electricity, 'would have been truly remarkable'.

The St Andrews researchers then went on to pose questions which many had asked but none had proved able to answer: 'How reliable is the archaeological evidence? How reliable are the chemical analyses? What metallurgical techniques were available at that date? Is it possible to prepare an aluminium alloy by any of them? If an aluminium alloy was prepared, was it by design or by accident?'

Their discussion of the archaeological evidence did not detain them for long. They argued that standards of excavation are high in China - the painstaking manner in which the first emperor's terracotta army has been uncovered and preserved at Xian is one of the most recent examples - and concluded that 'there can be little doubt that the aluminium artefacts were found in the tomb'. They also gave short shrift to a suggestion that the belt had somehow been 'planted' by grave-robbers:

It is difficult to see why they should have left the silver objects in place and have carefully inserted pieces of aluminium for the confusion of future excavators. A tomb-robber is scarcely likely to have had scraps of kitchen utensils about his person and to have discarded them accidentally. It would also need a miraculous breeze to replace the dust.

Butler and his colleagues devoted most of their paper to the central question: Did the Chinese of the Jin Dynasty have the know-how to produce aluminium? While the modern method of isolation uses electricity, aluminium has been produced in furnaces, though these need to be extremely hot. The Chinese certainly had furnaces capable of producing high temperatures, perhaps as great as 1,500ºC, but, the St Andrews team concluded, these would have been capable of making metal containing only very small amounts of aluminium. And there was no method available to Jin Dynasty metallurgists which would have enabled them to manufacture aluminium of the purity of the metal found in Zhou Chu's tomb.

So what is the answer? How can it be that pieces of almost pure aluminium should turn up in an ancient tomb? With a touch of the theatrical, the Scottish researchers saved their theory for the last paragraph of their report:

We are led to suggest, for want of something better, [they wrote] that the aluminium was introduced as an academic prank by a participant who was probably greatly embarrassed when he realized the consequences of his actions. Fortunately for scholars in the West, the Chinese themselves were the first to doubt the authenticity of the claims. It is perhaps a mark of our regard for the enduring genius of the Chinese people that the claims were taken seriously for so long.

The St Andrews paper seemed to have settled the argument. Hoaxes, of course, are nothing new in archaeology, and the story of the Nanjing belt was duly dubbed 'the Chinese Piltdown' after the most celebrated hoax of modern times, in which a weird amalgam of a human cranium and an orang-utan's jawbone, unearthed in the south of England in 1912, was successfully passed off for some forty years as the skull of an important 'missing link' in the evolutionary chain.

Yet in 1985 the Bulletin of the Chinese Academy of Geological Sciences reported a discovery which revived the possibility that aluminium had been available in China at the time of General Zhou's burial. Geologists from the Shenyang Institute of Geology announced that they had found grains of 'native aluminium' in Guizhou Province. 'Native aluminium' is extremely rare, indeed only a handful of claims for its discovery have ever been made. According to the Chinese geologists, their specimens contained 'some copper and sulphur, also chromium and iron' and were harder than pure aluminium, but they were satisfied that they had been found 'in a situation where contamination by men was eliminated'.

The report brought this comment from Dr Anthony R. Butler of the St Andrews team:

I think the evidence for the presence of native aluminium is good but the manner of its production is obscure. The grain size indicates that it could not possibly have been used to make the Nanjing belt. For native aluminium to have been used for that, an even rarer geological process, giving lumps of aluminium rather than grains, would be necessary. While this is a possibility, made more possible by the discovery of grains of native aluminium, it remains a remote hypothesis. However, a general rule is never to underestimate the Chinese. After all, they did invent the compass, printing and gunpowder.

The Stone 'Doughnuts'

The achievements of the ancient Chinese were also much discussed in the course of a controversy that arose after the discovery of mysterious artefacts off the coast of southern California.

In 1973 a US Geological Survey ship dredged up a peculiar stone from 13,000 ft (4,000 m) down on the bed of the Pacific, off Point Conception. Roland von Huene, the geologist who first examined it, soon noticed something odd: the stone had a hole in the middle, and 'had clearly been made by tools'. The underwater 'doughnut' was covered with manganese deposits, which suggested that it had been on the ocean floor for some considerable time.

The 'doughnut' was quite a curiosity and when, two years later, a whole hoard of similarly worked stones were located off the same coast, historians of the sea really began to get excited. Two professional divers, Wayne Baldwin and Bob Meistrell, had been exploring a reef off the Palos Verdes peninsula when they saw at least twenty stones lying 16 ft (5 m) down amidst the seaweed. They brought a few to the surface and stored them in a yard outside a diving shop at Redondo Beach, south of Los Angeles.

The discovery of a few old stones does not often make the headlines, but the theories advanced to explain the purpose and presence of these 'doughnuts' off the Californian coast became big news. The stones, the theorists argued, were ancient ships' anchors of a type often found in the Mediterranean Sea. Sailors used to bore a hole in a heavy rock, tie a rope through it and cast this primitive anchor overboard when they wanted to moor their vessel. But it was the explanation advanced by a group of Californian academics, that sent the reporters rushing to their typewriters.

The stones, they opined, were anchors lost from Chinese ships which had visited America 1,000 years or more before - centuries before Columbus. James R. Moriarty III and Larry Pierson expressed little doubt. 'Stylistic comparisons with historical, archaeological and ethnological data indicate great antiquity for the anchors,' they wrote. 'Geologic studies show that the stone from which they were made is not of Californian origin ... It seems clear to us that Asiatic vessels reached the New World in pre-Columbian times.'

The idea that ancient seafarers reached America before Columbus is nothing new. Claims that the Chinese got there first rest on an account in a history of the Liang Dynasty, which nourished from A.D. 502 to 557. In 502 a monk called Huishen appeared at the court of the Emperor Wu Ti and told of a journey he had made to a wonderful country called Fusang. According to the mariner monk, it lay 20,000 li (6,500 miles) to the east of China.

Some later scholars, particularly those of the mid-nineteenth century, discovered in Huishen's narrative what they took to be uncanny similarities between Fusang and America, and on this somewhat slender evidence the idea that the Chinese had sailed in their junks to the New World took hold. It even survived a thorough debunking by Gustaaf Schlegel of the University of Leiden, Holland, in 1892. Schlegel argued that since Huishen's story clearly exaggerated some of the marvels he had seen - he had told of mulberry trees thousands of feet tall and of silkworms 7 ft (2 m) long - his estimates of the distance he had travelled to reach Fusang might well be inaccurate too.

From his description of the features of the country, Schlegel deduced that the fabulous Fusang had really existed, but was much closer to China than Huishen had admitted. Schlegel identified it as a large island near Japan called Sakhalin.

The discovery of the 'Chinese anchors' off California, however, revived the diffusionist arguments. In 1980 support for the American theorists who believed the anchors were evidence of a pre-Columbian landfall off California came from China itself- in the form of an article written by a leading maritime historian, Fang Zhongpu. According to Fang, 'many Chinese historians believe that the Fusang the monk Huishen had visited is today's Mexico', and he welcomed what he deemed to be this proof of 'friendly intercourse between China and America in ancient times', and argued that Chinese junks and seamen of 1,500 years ago were well capable of crossing the Pacific.

Meanwhile, however, analysis of the Palos Verdes stones by the Geology Department of the University of California at Santa Barbara, dealt a blow to the diffusionists' claims. The analysts found that the stones had not originated in China, but had come from the local Monterey shale.

Professor Frank J. Frost of the same university had been sceptical of the claims made for the Chinese, and he seized on the geologists' report in a bid to clear up the mystery.

Presumably someone already in California shaped these stones and drilled holes in them. Both the large number of objects (about 20) and the wide distribution over more than an acre of ocean bottom would seem to rule out any conceivable pattern left by a shipwreck. Instead, the impression left is of an area where boats anchored frequently and occasionally lost their anchors. The question remains, therefore, what frequent visitors came to this reef and anchored using primitive weight anchors made of local stone?

Frost soon found an answer: the Chinese. Not the early Chinese, however, but immigrants from the Pearl River delta who had settled in California in the nineteenth century and had started a flourishing fishing industry. They had brought with them the technology of their native land, and sailed the California coast in traditional junks and sampans.

To moor their vessels, they probably used the same kind of anchors as their forefathers - stones with holes bored in them. 'It is hard to resist the working hypothesis that the Palos Verdes stones represent evidence of nineteenth-century California Chinese fishermen who made frequent visits to a reef rich in marine life,' wrote Frost. He added: There is no other human agency in the history of the California coast that had both a need for implements made of local stone and the means to get them where they are found today.'

Professor Frost's arguments are rational and convincing, but can they be said to solve the 'Chinese anchor mystery' once and for all? There can, of course, be no definitive answer. The methods of the Chinese fishermen of California died with them and, as Professor Frost observes, 'unfortunately, a hundred years ago other Californians were more interested in driving the Chinese fishermen out of business than in studying their technology'. There is still a remote possibility that Chinese mariners did visit America before Columbus and fashioned anchors out of local stone, but if a mystery can be explained simply, it is perverse to settle for any more outlandish solution.

The Nazca Lines

The Nazca lines of Peru offer a more profound challenge to archaeologists, but recent research in laboratory and desert has brought a new understanding of their purpose. Theories that the lines were runways for alien spacecraft or tracks laid out for pre-Columbian athletics meetings have long been laughed out of court.

Anthony F. Aveni, Professor of Astronomy and Anthropology at Colgate University in the United States, led a team which began an elaborate survey of the lines in 1977. Earlier, he and a colleague had studied the extraordinary ceque system of the Inca capital, Cuzco. This was a network of forty-one 'invisible lines' radiating out of the city. Each was punctuated at intervals by a huaca, or sacred place. Unlike the lines, these huacas actually existed, and the investigators found that many of those at the end of the ceques marked places near which water could be found. The system also operated as a gigantic agricultural calendar: each huaca signified a different day in the farmer's year, and some ceques pinpointed where the sun would be on important dates, thus signifying when, for example, the crops should be sown. The ceques were also used as ritual paths by pilgrims.

The Nazca lines were probably laid out 1,000 years before Cuzco was built. Were they, Aveni wondered, forerunners of the ceque system? To find out, six expeditions laboured under the desert sun; volunteers followed the lines for miles across the pampa; aerial photographers produced a photo-mosaic; and the triangles, trapezoids and spirals were meticulously measured. The results were fascinating. Like the spokes of a vast wheel, many lines radiated from centres, each of which took the form of a natural hill or a mound on which a rock cairn had been constructed. These centres reminded the investigators of the huacas of Cuzco.

Many of the Nazca lines, like their invisible counterparts at Cuzco, turned out to be associated with water. Some opened up into vast trapezoids, two thirds of which were aligned with water courses with their 'thin ends' pointing upstream. The astronomical studies added extra weight to the theory. The lines that intersected the part of the horizon through which the sun travelled in the course of the year tended to cluster around one particular area - the region where the sun appears in late October, a time especially important to the Nazca farmers, for this is when the dried-up rivers flow again with water. This suggests that the Nazca lines, like the ceque system, formed a giant agricultural calendar.

The survey also revealed that the lines had been used as pathways and established that they have many of the characteristics of the old Inca roads. Aveni speculated that workers might have used them to travel from one river valley to another and that the paths might have had some sort of ritual use.

This is how Professor Aveni summed up his findings:

To be sure, our argument has proceeded by analogy, but whatever the final answer may be to the mystery of the Nazca lines, this much is certain: the pampa is not a confused and meaningless maze of lines, and it was no more intended to be viewed from the air than an Iowa wheatfield. The lines and line centers give evidence of a great deal of order, and the well-entrenched concept of radiality offers affinities between the ceque system of Cuzco and the lines on the pampa. All the clues point to a ritual scheme involving water, irrigation and planting; but as we might expect of these ancient cultures, elements of astronomy and calendar were also evident.

Although the question of why the lines were built is the major mystery of Nazca, there is another intriguing enigma still to be resolved: How did the Indians of at least 1,000 years ago draw the birds, insects, and animals that make up the huge 'picture book' of Nazca? The outlines are difficult to make out on the desert floor, yet from the air their precision is flawless.

In August 1982, a small group of enthusiasts assembled at a location far to the north of the 'giant scratchpad', a landfill site near West Liberty, Kentucky. Joe Nickell of the University of Kentucky, an experienced investigator of mysteries, planned to work out how the vast drawings of birds, insects and animals that probably predate the larger Nazca lines were actually inscribed onto the desert. Maria Reiche, the stalwart investigator whose study of Nazca began in the 1940s, had noticed an important clue in the course of her painstaking mapping. The draughtsmen of ancient times had made small-scale preliminary drawings of the figures on plots 6 ft (2 m) square. They had then enlarged them, section by section. There can be no doubt that this was the method used: like the lines and figures, these sketches have survived the centuries and can still be seen.

Maria Reiche was less specific about how the drawings were scaled up, however. She suggested that the Nazca Indians could have used a rope and stakes to make straight lines and circles, but was vague about how they could have found the right positions for the stakes that served as the centres of circles or the ends of straight lines. Joe Nickell thought he might have the answer, and called in two of his cousins to put his theory to the test. They decided to try to reproduce one of the most striking of the Nazca drawings, the giant 440-ft (135-m)-long condor. Nickell wrote afterwards:

The method we chose was quite simple. We would establish a center line and locate points on the drawing by plotting their coordinates. That is, on the small drawing we would measure along the center line from one end (the bird's beak) to a point on the line directly opposite the point to be plotted (say a wing tip). Then we would measure the distance from the center line to the desired point. A given number of units on the small drawing would require the same number of units - larger units - on the large drawing.

Maria Reiche had suggested that the desert artists had used a standard unit of measurement known as the 'Nazca foot' - about 32 cm (12.68 in) long. So, using the 'Nazca foot', a wooden T-square to ensure each measurement they made would be at right angles to the centre line, a supply of tennis-court marker-lime for drawing the outline, and with an aeroplane standing by for aerial photography, Nickell and his group (which now included his father) set to work. The task took nine laborious hours of plotting and pegging. Over a mile of string connected the stakes, but the outline was unmistakable. After a week's delay, due to rain, they traced it out with lime, and the figure, 'possibly the world's largest art reproduction', could be photographed in all its glory from the air.

Cheerfully, Nickell summed up. They had proved that:

the drawings could have been produced by a simple method requiring only materials available to South American Indians centuries ago. The Nazcas probably used a simplified form of this method, with perhaps a significant amount of the work being done freehand. There is no evidence that extra-terrestrials were involved; but, if they were, one can only conclude that they seem to have used sticks and cord just as the Indians did.

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[See First Set Of Plates pl01 to pl26]

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The Peruvian Stone Walls

Professor Jean-Pierre Protzen, Chairman of the Department of Architecture at the University of California at Berkeley, also found that practical experimentation can help solve some mysteries. While on a visit to central Peru in 1979, he became fascinated by the ruins of buildings constructed by the ninth emperor of the Incas.

The huge blocks of stone, some weighing well over 100 tons, were put together in a most remarkable way: each block fitted together so tightly that it was impossible in many cases even to slide a knife-blade into the joints. What was the secret of the Inca stone-masonry? Professor Protzen assumed that when he returned to Berkeley he would be able to find out from a book in the university library. But there was no book, and no one seemed to know the answer.

In 1982 the professor set out for Peru once more, determined to crack the mystery. First he examined the spectacular Inca walls at Cuzco, Saqsaywaman and Oliantaytambo. Then he visited the quarries from which the stones had been cut, marvelling in passing at the slides down which the blocks were transported from the rock face. One slide, at Kachiqhata, had 'an awesome 40-degree slope with a 250-meter vertical drop'.

At another quarry Protzen found 250 large stones which, he realized, were 'examples of all the stages of production, from raw stone to finely dressed blocks'. Scattered amongst them were stones which had obviously come from outside the quarry, probably from the banks of the Vilcanota River nearby. These, he decided, were hammers with which the blocks had been worked. He identified three types: the heaviest probably shaped the stones immediately after they had been cut from the main rock face; the medium-sized ones could have been used for dressing the blocks; and the edges would have been fashioned with the smallest hammers.

The time for theorizing was over. The professor chose a likely looking block and set to. With just six blows of a 9 lb (4 kilo) stone hammer, he shaped a rough block, and then, with another hammer, pounded one of its faces until it was smooth. To protect it from the impact of heavy blows to the next face, he used one of the smallest stones to draft the edges before turning the block over. Ninety minutes later, three sides were dressed. A comparison of the test block with those worked by the Incas confirmed that Professor Protzen's hypothesis was plausible.

Now it was time to tackle the heart of the mystery. How had the Inca stonemasons managed to make the blocks fit together so tightly? The examination of ruined walls provided the clues. Joints - usually concave depressions - were cut in the lower blocks so that the upper course could fit into them precisely. The sides of each block were slotted together in the same way. Once again, Professor Protzen tried the technique himself. He took the block he had already dressed and placed it on top of another. Then he traced the outline of the upper stone upon the lower, removed the top block, and pounded away until he had hollowed out a depression for the top one to fit into. Before long, both stones were tightly locked together.

The enigma of Inca stone-masonry had yielded to an enquiring mind and an energetic arm.

The Great Glass Slab of Galilee

There are some mysteries, however, which cannot be explained by experiment or enterprise; when archaeologists have to make do with excavation and informed guesswork. In the 1950s the local authorities near Haifa, Israel, decided to build a museum in a cave at Beth She'arim, the site of an ancient city where Jews were buried in catacombs. The cave had been used as a water tank and, since it was badly silted-up, a bulldozer was brought in to clear it. In the middle of the floor, the machine struck a large slab, which the museum administrators decided to keep and use as the base of a display case. For years afterwards, visitors filed past, looking only at the model of a building laid out upon it, not realizing that the slab itself was by far the most interesting exhibit of all.

Eventually, some local archaeologists took a closer look at the slab. To their amazement, they found it was not made of rock at all, but of strange purplish-green glass. A search through the record books brought a further surprise. The slab measured 3.40 m by 1.94 m (over 10 ft by 6 ft), was about 50 cm (nearly 20 in) thick, and weighed about 8.8 tons. This made it the third largest piece of glass ever known to have been made by man, ranking only after two giant mirrors manufactured in 1934 - making full use of the current technology - for the Hale Telescope at Mount Palomar in the United States. Yet from what was known of the history of the cave, the 'Great Glass Slab of Galilee', as it came to be called, was well over 1,000 years old.

In 1964, soon after its discovery, a team of American experts was called in to investigate the slab. Its leader, Dr Robert Brill, Administrator of Scientific Research at the Corning Museum of Glass, New York, was mystified. The slab was certainly made of glass - laboratory tests on a core taken from it with a power-drill proved as much. But what was the third largest piece of glass in the world doing in a cave in Galilee? And who had made it, and when and why? To produce a slab that size would certainly have been a daunting task, requiring around 11 tons of raw material kept at a temperature of 1,150ºC for several days.

In fact, finding out how the glass had been made did not prove difficult. Brill and his colleagues managed to excavate beneath the slab and there they found a layer of large stones which appeared to have once been covered with clay. This, Brill guessed, was the bottom of a large tank in which the glass must have been mixed and fired. He went on to suggest that firing chambers were probably built at the side to provide heat for melting the material and that the whole tank was covered over. He later learned from the archaeologists that, within living memory, a course of large stones had existed around the slab, and this strengthened his conviction that a tank had been used.

It was fairly simple, too, to guess why the slab had been left in the cave: Brill's chemical analysis of the core showed that it had gone wrong in the furnace. At the bottom the raw materials had not fused together properly, the whole thing had crystallized - and its dull appearance meant that no one recognized it as glass when the museum was opened.

Comparisons with other ancient glass from the area - in addition to his knowledge of the history of the cave -enabled Brill to estimate that the slab had been made at some time between the fourth and early seventh centuries, but he was at a loss to explain why the glassworkers of ancient Galilee had embarked upon their laborious task in the first place. There was one clue, but it told him little.

Manganese had been added to the mixture to give the glass its purplish colour, suggesting that it had been designed to be decorative. Perhaps, Brill speculated, the slab had been destined to be broken up into small pieces and sent out to craftsmen in other villages, who would then have fashioned glass objects of their own from them. Or perhaps the intention had been to keep it in one piece and use it in a building as an impressive architectural feature. Robert Brill was doubtful that the answer would ever be forthcoming. He wrote:

We must in the meantime commend its unknown makers for their engineering skill. They brought over eleven tons of raw materials to a temperature in excess of 1000ºC for several days, and produced a glassy consolidated mass. This was a considerable technological feat, and I know of no similar accomplishment in the metallurgical or other pyrotechnic arts in ancient times.

Dr Brill's approach is realistic. There can be no definitive solutions to such mysteries, for, as far as we can tell, the ideas and technological secrets of so many ancient civilizations were not written down. The fact that the Californian Chinese anchors were thought mysterious shows how quickly know-how can be forgotten. Scientists can learn much about mysterious artefacts by experiment, the archaeologist's spade can turn up many clues, but the exact intentions and methods of the people who traced out the huge pictures of the Nazca desert, of the Galilean glass-makers, the Inca builders and the metalworkers who worked a Chinese warlord's belt are likely to stay lost forever in the tantalizing silence of the past.

Arthur C. Clarke comments:

I am happy to see a solution to the mystery of the 'ancient' Chinese aluminium belt, which has worried me for years. Technologically, such an artefact would be almost as anomalous as a medieval transistor radio. Of course, the solution may not be right, but it is highly plausible - and one possible solution is infinitely better than none. Recent scandals have shown that archaeological (and other scientific) frauds are by no means uncommon, and may begin as practical jokes that sometimes get out of hand. The classic case is that of the eighteenth-century German professor whose academic rivals carved amazing fake fossils for him to discover.

The fact that some of them appeared to be of fiery comets, stars and spiders spinning webs only increased his eagerness to publish his findings. By the time he discovered that he had been hoaxed, it was too late - and he spent the rest of his life (and fortune) buying up the volumes in which he had printed his revolutionary conclusions.

A few months ago I received a letter and photograph from Mr William W. Jenna of Bel Air, Maryland, containing information about an item which one might suspect of having a similar origin. Note that it was found on the high plateau near Nazca - where, as we have seen, some surprisingly ingenious technology once flourished!

About ten years ago, my wife and I purchased a piece of pre-Columbian pottery from a collector who had unearthed it in a dig in Peru in the late 1950s. The piece was a product of the Vicus culture, which flourished in Peru between the first and fifth century A.D. Because of the location in which the piece was found, the high plateau near Nazca, it was dated by museum experts at around A.D. 200; at least 1,500 years before the dawn of the industrial revolution. Yet it would appear to be an exact representation of a modern steamroller, complete with a front 'smoke-stack' and, even more startling, a cab with a driver inside, tire-treads, and wheel spokes. All this from a culture which, as far as we know, did not even make use of the wheel. Everything was carried or dragged from place to place, and it was not until the Spanish explorers conquered the area that wheeled vehicles (carts, etc.) were introduced.

Up to this point, only a handful of people have seen the piece, primarily museum curators, who, like ourselves, were both puzzled and fascinated by the piece. The thing is in mint condition, having fortunately been perfectly preserved by the extremely arid conditions of the high plateau; all the museum people agreed that it was a superb example of the Vicus double-vessel work, and all placed the date between A.D. 100 and 300.

The resemblance to a modern (well, 1930-ish) steamroller is certainly striking. But need it be more than coincidence? I'm prepared to admit that the makers of the Nazca lines had hot-air balloons, as has been ingeniously argued. But not steam engines.

The origin - and method of manufacture - of the stone 'Giant Balls' of Costa Rica (see Arthur C. Clarke's Mysterious World, Chapter 3) is still an archaeological enigma. Surprisingly, it turns out that nature can make almost perfect spheres of stone. Hundreds of specimens of up to 11 ft (3 m) in diameter have been found in Mexico. They appear to be of volcanic origin, and were formed some forty million years ago when a torrent of incandescent ash cooled and crystallized. Although many of these natural spheres are almost geometrically perfect, they lack the finish of the man-made ones - though James Randi has suggested that they may have inspired their production. And, one might add, greatly assisted it: any sensible sculptor starts with a piece of rock as near as possible to the shape he's aiming at.

I am indebted to my old friend Colin Ross for information about much smaller spheres (the 'Moeraki Boulders') which occur in New Zealand. These are concretions, i.e. masses that have 'grown' from the surrounding rock by chemical precipitation over immense periods of time. Some are up to 6 ft (2 m) in diameter, and range from perfect spheres to 'highly irregular and fantastic shapes'.

Natural spheres can also be produced when rocks are trapped in holes on the beds of rivers, and strong currents continually turn them over and over. I am grateful to Hubert Siemerling for this information; he tells me that farmers in the Alps used this method to make stone cannon-balls, so the process must be fairly swift.

Never underrate Mother Nature. In this case, she has come up with several solutions to a problem which at first sight seems insoluble.

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