Alaskan glacier starting to surge.

From the air, North America's largest glacier appears perfectly still, like a vast river of milk frozen in place. But the Bering Glacier in southern Alaska is actually speeding downslope. For the first time in 26 years, much of the glacier has started to surge, says Bruce F. Molnia of the U.S. Geological Survey in Reston, Va.

Surging occurs when a large fraction of a glacier accelerates, in some cases reaching speeds 100 times greater than normal. Glaciologists believe surging results when water buoys the glacier off the bedrock, reducing friction on the ice. Researchers first noticed evidence of the Bering surge in early June. Normally, this glacier moves at a rate of about 3 meters per day. Scientists are now trying to gauge its current speed and determine how much of it is surging.

The 191-kilometer-long glacier entered a similar phase of rapid movement in the late 1950s and again in 1965 through 1967. Prior to those events, it had surged periodically around 1900, 1920, and 1940, with evidence of previous episodes going back several centuries, says Molnia. The current surge could produce an abnormal number of icebergs and cause local flooding.