Deep desires in Antarctica and Greenland.

Laboring in the coldest spot on Earth, an international team of scientists has reached a new milestone in drilling through ice, providing a chronicle of climate going back nearly 400,000 years. This record, set in January at Russia's Vostok Station in Antarctica, won't stand for long, however. Several research groups plan to perforate Antarctica and Greenland further in the next few years, hoping to unlock fresh secrets about how Earth's climate works.

The drilling at Vostok Station, about 1,200 kilometers from the South Pole, goes back to the early 1970s, when Soviet crews extracted their first deep core at this site. Working with French and U.S. collaborators, the Vostok team began a fifth core in 1990 and reached a depth of 3,348 meters early this year.

"This is the deepest site in the world," says Jean Jouzel of the Laboratoire de Modelisation du Climat et de l'Environnement in Gif sur Yvette, France. He spoke last week in Baltimore at a meeting of the American Geophysical Union.

The latest Vostok core reaches back through four ice ages, each roughly 90,000 years long, and the intervening warm periods known as interglacials, which last about 10,000 years each. Scientists are eager to study the oldest glacial cycles because previous cores drilled at Vostok and in Greenland record only the two most recent ice ages.

Ice sheets and glaciers grow annually, layer by layer, trapping information about climate in more than a dozen ways. By measuring the ratios of oxygen and hydrogen isotopes in the ice, for example, researchers can tell how atmospheric temperature rose and fell through the millennia. Dust and ash layers reveal wind patterns and volcanic eruptions. Gas bubbles tell how concentrations of greenhouse gases have shifted.

The Vostok squad expects to renew drilling later this year, hoping to deepen the hole by another 300 m, which will bring it to within 50 m of the base of the ice sheet. The team plans to stop there to prevent polluting what appears to be a lake of water below the ice, says Jouzel. Scientists want to develop ways of sampling the water, which may contain ancient microbes that have been locked away for 500,000 years or more.

Later this year, nine European nations and Japan plan to begin a 3,300-m-long core at Dome C, 600 km from Vostok. After 3 years of drilling there, the team plans to establish a hole in Dronning Maud Land, Antarctica. Because of its proximity to the Atlantic Ocean, this site should help scientists match Antarctica's climate history with information gleaned from Greenland.

U.S. researchers are focusing on the other side of the continent, West Antarctica. Early next year, they plan to begin a 1,000-m-long ice core at Siple Dome, upstream from the Ross Ice Shelf. Meanwhile, a Danish crew is scheduled to start a deep core in central Greenland this summer to resolve debate about the next-to-last interglacial, 115,000 years ago.