In one area directly above a subglacial lake, the ice fell by more than ten feet (three meters) over the course of 16 months, from late 1996 to early 1998, Wingham's team found.
The ice elevation normally changes less than a tenth of that amount each year.
Over the same period in two neighboring areas 180 miles (290 kilometers) away, also above lakes, the ice rose by three feet (one meter) or more.
The only explanation, the researchers say, is that, in a process not yet completely understood, water suddenly rushed out of one lake, ran beneath the ice sheet, then pooled in other lakes.
The lake that lost water is about 230 square miles (600 square kilometers) in area, a bit larger than California's Lake Tahoe.
Thus, the researchers estimate, the water must have flowed out of the lake at a rate that was, on average, about equal to that of a major river, such as London's Thames.
Raising the Stakes
"The chances are that what we've seen is a very common process," Siegert said. "We just haven't seen it before."
Other researchers agree that the new findings increase the risk of contamination spreading between lakes.
"We have to assume in every case that this risk exists and work to address it," said microbial ecologist Cynan Ellis-Evans of the British Antarctic Survey.
Many scientists suggest probing one of the smaller subglacial lakes before hitting Lake Vostok and have urged the Russian team to hold off.
The critics recommend waiting until scientists have developed better equipment and gained a better understanding of the subglacial systems.
"This is why we are taking so much time to get into a subglacial lake," Ellis-Evans said.
Microbiologist John Priscu of Montana State University in Bozeman agrees that plans to drill into the lakes have to take account of connections between the lakes.
The new findings also indicate that "subglacial lakes play a larger role in polar regions than previously thought," Priscu said.
Some areas in Antarctica show evidence of catastrophic floods in the past. One such spot is the Labyrinth, where deep trenches are carved into Wright Valley, a relatively ice-free area of the continent.
Trenches like those in the Labyrinth would require far bigger flows than those that Wingham and colleagues found in their study. But it is possible that huge subglacial lakes dumped their waters in the past, Siegert and Priscu say.
"To cause some of these features, a large amount of meltwater must have been released," Siegert said. "The only candidates are subglacial lakes."
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