Hidden in one of the upper layers of this mass, corresponding to the Pleistocene epoch, are the remains of an estimated 150 million mammoths. Some are frozen whole, as if in suspended animation, others in bits and pieces of bone, tusk, tissue and wool.

Woolly mammoths are the last of three extinct elephantine species that inhabited Siberia. They appeared about 400,000 years ago and lasted at least until 3,600 years ago - the age of some mammoth remains found on an island off the northern coast of the Russian region of Chukotka in 1993.

The tusks emerge with the spring thaw or after heavy rains, or along the eroding banks of rivers. A boom in gas and oil investment has added another source, as crews dig wells and dig ditches for pipelines. Fresh from the permafrost, mammoth ivory is nearly pristine, though with a characteristic green patina. But if left outside and exposed to the elements, it will disintegrate within three years into worthless splinters.

That is another point in favor of the mammoth ivory trade, said Tikhonov, the Russian mammoth expert. It encourages the gathering of tusks that would otherwise be lost. In fact, he said, vastly more mammoth ivory is destroyed in this natural cycle in Russia than is ever found and sold, perhaps hundreds of tons a year.

Gary Haynes, a paleoanthropologist at the University of Nevada at Reno, said he could not help but cringe at seeing the tusks destroyed. The growth rings and possible prehistoric human butcher marks contain a wealth of data on the ancient climate and peoples of Siberia that could shed light on, among other things, the debate over whether climate change or over-hunting, or both, felled the mammoths.

"There's a kind of discomfort when you're a scientist and you see something that could have scientific value being carved up and destroyed," Haynes said. "But this is the trade-off," he added. "I see the businessman's arguments, too. Mammoths are already extinct and people need an economy." In addition, the Russian government examines the tusks to make sure none bearing disease, prehistoric human markings or other scientifically valuable elements are exported.

About 90 percent of the Siberian ivory is exported to Asia, where it is principally used in the manufacture of personal seals that in Japan, China and Korea are used in place of signatures for business transactions. These seals support much of the Siberian industry.