Barri Jones, professor of archaeology at Manchester University, returned from Oman last week, having made several visits to the region at the invitation of the Omani government.
'This is undoubtedly a major archaeological find,' he said. 'The site is in the area known as Ubar and could well be the ancient city of Ubar itself.'
Until archaeologists began excavating the site last year, explorers had believed the ruins, at a small village called Shis'r, were no more than a few centuries old.
But radio-carbon dating has confirmed that the site is older than AD 200 and flint artefacts found there go as far back as 5,000 BC, according to Juris Zarins, professor of archaeology at South West Missouri State University. 'Some of the pottery may be 1,000BC to 2,000BC.'
Sir Ranulph Fiennes, the writer and explorer, and Nicholas Clapp, an American film maker, discovered the importance of Shis'r with the help of satellite maps of the area which revealed an ancient road leading from the village.
Initially it was thought the road led from Shis'r to Ubar, but Professor Zarins suggested it might be the ancient route from Ubar through the desert to the north. 'I said I don't think there is a site in the desert. Nobody is going to build a town in the dunes,' he said.
Professor Zarins uncovered a much bigger structure at the village, buried under the sand around a stone tower that has been used as a fort for the past few hundred years.
He found that the tower forms one corner of a much larger pentagonal-shaped enclosure protected by a wall fortified by eight other towers of various shapes and designs. Professor Zarins thinks he has found the ancient gate leading into the inner sanctum, which could be the oldest part of the city.
The team found a chess set, believed to date from the 10th or 11th century, in one of the towers and ancient fireplaces in another part of the ruins. Ubar was at the centre of the ancient frankincense trade and a regular array of stone struts sticking out from the walls are believed to be the anchor points for the tented market stalls where the sweet-smelling resin was sold.
The walls appear to have been rebuilt over many centuries and it is difficult to say how old they are, he said. 'I would say they go back at least to 1,000BC,' Professor Zarins said.
Ubar was first documented by the Greek geographer Claudius Ptolemy in the second century BC and is celebrated in The Thousand and One Nights.
Professor Zarins said: 'We began to find pottery that goes back to Roman times. We didn't find pottery this old in any other place so I said it's got to be Ubar. The site fitted all the descriptions of the things we needed.'
The site would predate many of the famous cities of antiquity, such as Troy.
The ruins at Shis'r sit above the last source of 'sweet' water before merchants and nomads of ancient times would have entered the 560-mile wide desert of the Empty Quarter. Professor Zarins and Professor Jones believe the site would have been the bottleneck for the vast caravans on their journey from the Qara Mountains to the south to Mesopotamia.
Professor Zarins is also excavating another site, at Ain Humran on the other side of the Qara Mountains not far from the Omani coast. That is larger than Shis'r and could be the city of Sopphar Metropolis - the 'mother city' documented by Ptolemy.
The archaeologists say there is evidence that the site at Shis'r lost its importance around the second or third century AD, which would have coincided with the collapse of the underground limestone cavities in the region, caused by the gradual lowering of the water table over the centuries.
Sir Ranulph suggested that this ties in with the Koranic description of the fall of Irem, a wealthy city destroyed by God because of the wickedness of its citizens.
Irem, according to the Koran, was 'adorned with pillars the like of which had not been built within these lands'. Professors Zarins and Jones believe it is impossible to say whether Irem and Ubar are the same place.
They are equally sceptical of claims that Ubar is one and the same as the city of Emporium Omanum (Marketplace of Oman) mentioned in the ancient Periplus document written by a Greek-speaking sea captain in the first century AD.
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