The 200,000-year-old find proves for the first time that the giant woolly mammoths of the Ice Age flourished in warm as well as cold climates.
A two-acre area crammed with the remains of hundreds - possibly thousands - of the animals has been found near the village of Eynsham, seven miles north-west of Oxford.
So far several small trenches covering a total of 20 square metres have yielded the remains of at least nine mammoths, and possibly as many as 20.
Dr Kate Scott, a palaeontologist from Oxford University, and her team have unearthed 12 perfectly preserved mammoth tusks, 8 mammoth teeth and 60 mammoth bones.
The excavations, funded by the British Academy, the Society of Antiquaries, the Leakey Foundation - an archaeological research funding body - and Earthwatch - the environmentalist organisation - have also produced fossilised remains of horses, bison, and possibly giant cattle known as aurochsen, as well as evidence of early humans.
Three stone hand-axes, at least one of which dates from the same period as the mammoths and other fossilised animals, have been unearthed.
The discovery is extraordinary not only on account of its scale, but also because it settles an academic debate as to precisely what sort of environments mammoths lived in. The excavation is proving for the first time that woolly mammoths were able to flourish in a warm environment as well as the semi- arctic one they are more usually associated with.
Environmental evidence found on the site, freshwater mussels, specific types of beetles and plant remains all point to a climate at least a couple of degrees warmer than southern Britain has now.
Painstaking examination of the material being excavated is producing the remains of hundreds of different species of animals, fish, plants and insects, including perfectly preserved flies, complete with wings.
The finds so far suggest that the warm weather may have had an extraordinary, though probably indirect, effect on the mammoths.
All their tusks are unusually big - the largest being 9ft (2.7m) long - while their teeth are on average about 15 per cent smaller than those of other mammoths discovered around the world.
But the question as to why so many mammoths died in such a small area remains a mystery.
The most likely explanation is that the animals were caught in a flash flood - or series of flash floods - got bogged down in mud and drowned.
The site is strung out along what used to be the course and flood plain of an ancient river and the superb quality of the tusks and bones suggests that they were rapidly buried beneath mud and gravel and did not roll around within the water for any length of time.
If the density of remains is constant across the site there could be 5,000 mammoths there.
The presence of stone hand-axes also suggests that a human encampment could lie nearby. Because of the sight's waterlogged state any such human camp site might well be perfectly preserved with wooden and other normally perishable tools surviving as well as the usual array of stone artifacts.
If such an early human encampment is discovered in perfect condition, it would be the first such example of this antiquity to be found anywhere in the world. The site predates the development of anatomically modern humans by about 100,000 years.
The people who lived there would have been early examples of the Neanderthal species which inhabited western Europe.
The excavation is being carried out in a disused quarry earmarked for development as a rubbish dump, possibly later this year.
However the mammoths and Stone Age discoveries are likely to turn the quarry into a palaeontological and archaeological site of world importance which may take several years to excavate.
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