UNCOVERING EVIDENCE OF GIANT ANCIENT ICE SHEETS.

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A scientific expedition on a submarine in the Arctic has found the footprints of ancient floating ice sheets--possibly the largest masses of ice ever to cover the Earth's oceans. Studying the formation and demise of these ancient ice sheets may help scientists better understand Earth's climate changes and, in particular, predict whether the melting of today's polar ice could lead to catastrophic floods in the future.

Leonid Polyak, a research scientist at Byrd Polar Research Center at Ohio State University, Columbus, and his colleagues obtained sonar images of the Arctic Ocean floor through a unique collaboration between the U.S. Navy and civilian scientists--the Science Ice Exercises (SCICEX) program. Within two separate, somewhat elevated regions of the Arctic Ocean floor--the Lomonosov Ridge near the North Pole and the Chukchi Borderland near Alaska--SCICEX images showed numerous features carved into the seafloor, including matching sets of parallel grooves and ridges. Sometime in the past, the bottom of a very massive floating ice sheet scraped across the seafloor in both area--almost two-thirds of a mile below the water surface at the Lomonosov Ridge and more than 2,200 feet below the water surface at the Chukchi Borderland.

The sonar images clearly showed objects resembling rocks and other debris that may have once been dragged along the seafloor beneath the grounded ice. "Such amazingly coherent sets of streamlined grooves and ridges could only be made by one thing--sliding ice," Polyak explains. Only a large ice sheet could carve such a broad set of parallel features. Free-floating icebergs carve random patterns into the seafloor.

The finding may bolster a theory held by some scientists that one giant ice sheet covered the entire Arctic periodically during the ice ages that...