In fact, the Nenets' underworld tales are right: The Siberian subsoil teems with woolly mammoths. At ice-out each summer, hundreds of their tusks, other teeth, and bones appear on the banks of rivers and lakes and along the seacoast, freed by erosion from the frozen ground where they have lain for tens of thousands of years. Since the botanist Mikhail Ivanovich Adams recovered the first woolly mammoth carcass in Siberia in 1806, about a dozen other soft-tissue specimens had been found, including several calves ranging in age from newborn to about a year. Yet no carcass of any age was as complete as the creature Yuri Khudi had found—and now lost—on the Yuribey River.

In the time of the mammoths, the landscape over most of their range looked very different than the barren heaths and boggy tundra surrounding the river today. The air was drier, cloud cover was limited, and strong winds swept the electric blue skies. In place of tundra grew a vast, arid grassland that paleobiologist R. Dale Guthrie has called the mammoth steppe, stretching from Ireland to Kamchatka and across the Bering land bridge to Alaska, the Yukon, and much of North America. The grasses, broad-leaved herbs, and low shrubs of the steppe provided nutritious food, and in addition to mammoths, nourished a profusion of other outsize, exuberantly hairy mammalian megafauna—woolly rhinoceroses, enormous long-horned bison, and bear-size beavers, as well as the fearsome carnivores that hunted them: saber-toothed cats, cave hyenas, and giant short-faced bears.

Then, between 14,000 and 10,000 years ago, the mammoths disappeared from most of their range, along with most of the other large mammal species in the Northern Hemisphere—as many as 70 percent in some regions. These extinctions were so sweeping that scientists have evoked a number of cataclysmic events to explain them—a meteorite strike, killer fires and droughts, and a virulent, cross-species hyperdisease. Since the extinctions coincided with the end of the most recent ice age, however, many researchers believe that the primary cause of the great die-off was the sharp rise in temperature, which dramatically altered the vegetation. A recent computer simulation of landscape changes during the late Pleistocene suggests that 90 percent of the mammoth's former habitat disappeared. "We have strong evidence that climate change played a significant part in their extinction," says Adrian Lister, a paleontologist and mammoth expert at the Natural History Museum in London. "In Eurasia, the timing of the two events matches closely."

The extinctions also coincided, however, with the arrival of another ecology-altering force. Modern humans arose in Africa about 195,000 years ago and spread into northern Eurasia some 40,000 years ago. As time went on, their expanding populations brought increas­ing pressure to bear on prey species. In addition to exploiting mammoths for food—a big male killed in the autumn would see a band of hungry hunters through many lean winter days—they used their bones and ivory to make weapons, tools, figurines, and even dwellings. Some scientists believe that these human hunters, using throwing spears fitted with deadly stone points, were as much to blame as climate change for the great die-off. Some say they caused it. The debate over the megafaunal extinction is one of the liveliest in paleontology today, and not one likely to be resolved by a single specimen, no matter how complete. But Khudi was right that the now missing baby—its flesh, internal organs, stomach contents, bones, milk tusks and other teeth, all intact—would be of enormous interest to the outside world.