The masterpieces discovered in a French cave will help unravel the myst eries of Stone Age art, says David Keys
It was a tiny draught of warm air emanating from a pile of loose rocks that led to the greatest prehistoric art find for half a century. Jean Marie Chauvet, a speleologist employed by the French Ministry of Culture, was carrying out an inspection of existing cave sites in the Ardeche, north-west of Avignon, when he quite literally stumbled across the entrance to the finest complex of caves uncovered since the Lascaux caves in the Dordogne were discovered 55 years ago. As a chance find it was all the more remarkable because this is not an area that is particularly rich in large caves.

Gaining entry to the complex, at the foot of a cliff near Vallon-Pont-d'Arc, was a daunting experience. Chauvet and his colleagues slowly removed the rubble through which the air was coming until a tiny passageway 8ft long high and slightly over a foot wide was revealed. Breathing in, they wriggled through the gap on to the roof of the first gallery. A rope ladder was then needed to descend 30ft to the cavern floor.

What they found astonished them. Some 18,000 years ago the caves were cut off from the outside world when a landslide blocked the main entrance. When Chauvet and his colleagues shone their torches into the darkness, they realised not only that the walls were covered with paintings but also that the complex had been left exactly as it was when the Stone Age painters last used it. They were the first human beings to enter the caves since the Ice Age.

Chauvet returned to the site with Jean Clottes, a French cave art expert, last month. When Clottes entered the cave and found himself face-to-face with a wall covered with magnificent painted horses' heads, wild oxen and two fighting rhinos, he felt "a surge of admiration".

"These works of art were made by the Leonardo da Vinci of the Ice Age," he said, with great emotion. "I felt I was standing in front of some of the great artistic masterpieces of mankind."

The cave is magnificent. Huge stalactites and stalagmites, some formed into the most bizarre shapes, loom large. At the sides of the caves Stone Age hearths survive. In the centre of the main chamber what appears to be a kind of altar still has a bear's skull placed on top of it.

And on the soft cave floor are a myriad of footprints of the Stone Age tribes people themselves - footprints which appear as fresh as the day they were made some 18,000 years ago.

This is the first time that modern archaeologists have found a cave that is both artistically spectacular and also in pristine condition. This unique combination gives them an enormous nudge towards cracking one of prehistory's greatest mysteries - the meaning and function of Stone Age cave art.

On the walls of the French caverns are paintings and engravings of at least 13 different species of animal - a greater variety than any other Ice Age cave. In total the cave explorers have found at least 250 images so far, most of them executed in black and red pigments, and several hundred more probably still await discovery. In the complex's four chambers are highly naturalistic paintings of more than 40 woolly rhinoceroses, a similar number of lions and bears, together with at least 20 horses, wild oxen and bison. Also portrayed are reindeer, mammoths, ibex, giant deer, a leopard and a hyena.

There is also an engraving of what appears to be an owl and at least a dozen red ochre images of human hands. Some of the animals portrayed are unique to these caves - the hyena, leopard, rhinoceroses and owl - and the quality of the work is remarkable, with shading of the black pigment allowing the yellows and whites of the underlying rock to show through. Other engravings of various animals adorn a wall which has not yet been reached, and other areas are likely to yield yet more masterpieces.

An investigation of the soft cave floor - unaltered for 180 centuries - will probably yield the information of most value. Examination of the hearths should reveal what animals, if any, were being cooked, and whether they include those portrayed in the art; the footprints should tell whether women and children were there or only adult men, and whether they were walking, running or perhaps even dancing.

Microscopic examination of flint implements found in the complex will reveal what the tools were used for. A technique known as "use-wear" analysis should detect tiny tell-tale scratches on the flint surfaces, which will show whether animals were being cut up. Testing the blood residues on flint blades will provide evidence as to which creatures were being killed.

A search for Stone Age fingerprints preserved on the paintings could also yield vital data. An analysis of large numbers of different fingerprints could yield clues as to the ethnic identity of Europe's Ice Age population. The archaeologists will also behunting for scraps of human skin or hair that may have survived and could yield DNA genetic material.

All of this, however, is some way in the future. For the moment archaeologists are content to have stumbled on what amounts to a Louvre of prehistoric art.

Stone Age art is extraordinary not only because of its great naturalistic beauty but also because of the time in which it flourished and the mysterious fact that it died out utterly. The world's first known naturalistic art flourished in what is now Germany some 33,000 years ago at the beginning of the last Ice Age, and died out - in France - around 12,000 years ago, at the end of the last Ice Age. For 8,000 years the world then appears to have had no naturalistic art whatsoever until it was reinvented in a totally different form in the Eastern Mediterranean about 4,000 years ago.

Early naturalistic art - the sort of paintings made famous by the caves of Lascaux in France and Alta Mira in Spain - therefore seems to have been inspired quite specifically by the Ice Age and its challenging climate. Not only does the start and demise of the art style coincide with the commencement and end of the Ice Age, but the peak of the style, both in terms of quantity and quality, coincides exactly with the peak of the glaciation, when the cave painters of France would have been hemmed in by local ice caps on the Pyrenees, the Alps and Massif Central.

This year archaeologists will carry out dating tests and analysis of the pigment used in the paintings - they expect that the red is ochre but will be interested to know whether blood was used. They will also construct a system of paths and gangways to enable a proper survey to take place.

In 1996, a survey and research work will start in earnest - and could last for at least 30 years. There are a quarter of a mile of galleries to explore and record, and it will all have to be done without disturbing most of the soft cave floor.

For the time being, the function of Europe's cave paintings, located in hundreds of cave complexes - there are 130 in France alone - remains a mystery. Certainly the masterpieces were of religious and social importance. They may represent gods or ancestral spirits, and perhaps the caves were the scenes of complicated ritual and initiation ceremonies. Whatever its symbolism the art would have played some role in welding together society at a time when climatic and therefore general conditions were worsening.

Exactly how the art performed this role and what the religious and social beliefs really were remains a mystery - but it is one which this French cave could eventually crack.