Cold and deep: Antarctica's Lake Vostok has two big neighbors.

Trapped beneath Antarctica's kilometers-thick ice sheet are two bodies of water that rival North America's Great Lakes, new analyses suggest. The geological setting of these huge, unfrozen lakes hints that they may harbor ecosystems that have been isolated for millions of years.

More than 140 lakes lie buried beneath varying thicknesses of Antarctic ice, but most of them are small and shallow, says Michael Studinger, a geophysicist at the Lamont-Doherty Earth Observatory in Palisades, N.Y. Lake Vostok, discovered decades ago, is the largest. It's the size of Connecticut and holds 5,400 cubic kilometers of water, enough to fill Lake Michigan.

Scientists who've drilled through Lake Vostok's overlying ice sheet to within 120 meters of the lake's upper surface have found microbes trapped in the ice (SN: 10/9/99,p. 230). The researchers view that finding as a tantalizing clue that the lake may hold a thriving ecosystem.

Lake Vostok sits in a basin that formed as Earth's crust stretched thin, a feature that had set this body of water apart from all other subglacial Antarctic lakes, says Studinger. Now, he and his colleagues have used a collage of data to depict two large subglacial lakes near Lake Vostok and to determine that they also sit in basins formed by a thinning tectonic plate.

One of the lakes is dubbed 90[degrees]E because it stretches along that longitude. The other is called Sovetskaya, after the Russian research station atop it. Although scientists knew of these two lakes, they had no notion of their sizes until they saw recent satellite images of the region, says Studinger.

90[degrees]E Lake has a surface area of about 2,000 square kilometers, about half the size of Rhode Island, which makes it the second-largest known subglacial lake in Antarctica. It probably holds about 1,800 [km.sup.3] of water, more than enough to fill Lake Ontario. Sovetskaya Lake covers about 1,600 [km.sup.2]. Studinger's team describes the lakes in the Jan. 28 Geophysical Research Letters.

Ice-penetrating-radar data gathered during aerial surveys indicate that the upper surfaces of these lakes lie beneath 4 km of ice. A new analysis of measurements of Earth's gravitational field suggests that the lakes in some places are about 900 m deep.

The lakes remain unfrozen because heat seeps up from Earth's interior and insulating blankets of ice lie above them, says Studinger. Any ecosystems now in the lakes would have been isolated from Earth's surface for 35 million years, the estimated age of the ice sheet in that region.

Because of their great sizes, the covered lakes probably have always contained at least some liquid water, says David M. Karl, an oceanographer at the University of Hawaii in Honolulu.

"This is an important discovery," says Karl. "It shows how little we know about the Earth around us"