An iceberg almost the size of Rhode Island is close to breaking out of Antarctica's Weddell Sea into the open ocean, where it might pose a serious threat to the supertanker traffic moving around Cape Horn at the tip of South America.

"This is the largest iceberg we've ever tracked," Lt. Cmdr. Tom Nelson of the U.S. Navy's Fleet Weather Facility said the other day. "It's just a little smaller than Rhode Island, the biggest piece of ice we've ever seen."

Measuring 45 miles long and 25 miles wide, the iceberg is so big that two summers ago it rammed the larsen Ice Shelf on the coast of Antarctica and broke off a second iceberg about half its size, which itself has begun to drift slowly toward the open ocean. The smaller iceberg is about 60 miles behind the big one.

"They're both moving in a clockwise way," Nelson said from his office in Suitland, where the Fleet Weather Facility keeps track of sea ice and icebergs in both polar regions. "And unless they make another circuit of the Weddell Sea they'll move into the South Atlantic where they'll move into the South Atlantic where theyll almost surely break up in a couple of years" because the ocean waters are much warmer than Antarctic waters.

Both icebergs are in water right now that is two degrees below zero Celsius, while the ocean waters just to the northeast are 12 degrees warmer than that.

"Neither iceberg would be a threat now becaust they're so massive and easy to see," Nelson said. "But when they start to break up they break up very rapidly and that's when they become dangerous."

Nelson said the Navy has been watching the giant iceberg for the last 10 years, ever since it broke off the Princess Martha coast of Antarctica. The iceberg came loose from the ice shelf either because of prolong 100-mile-an-hour winds, a collision with another iceberg or both.

Old maps show the giant iceberg as a tongue of ice extending out from the Princess Martha coast. The tongue is no longer there.

The Navy has tracked the iceberg with the aid of satellites, which periodically photograph it.

NASA's Landsat satellite and two weather satellites provide the pictures. One of the weather satellites (Nimbus) photographs it through the long polar night and during storms with the aid of an infrared camera.

Currents have slowly taken the iceberg northeast through the Weddell Sea, though the berg has been grounded twice. It stayed grounded for five years on one occassion, finally being pulled loose by 150 knot winds two years ago.

"That's how we found out the iceberg is almost 1,000 feet thick," Nelson said, "because it was grounded in water that's almost 1,000 feet deep."

The second time the iceberg ran aground was last year, but it only stayed grounded for about two months. Only one end was aground that time, which Nelson said allowed the Navy to watch it, in photographs, rotate like a pinwheel. It came loose after a storm.

Big as it is, the iceberg, like all Antarctic icebergs, resembles a table. Arctic icebergs look like ice mountains reaching out of the sea.

Since icebergs were formed from the polar ice sheets, they are made only of fresh water. Nelson said the iceberg floating through the Weddell Sea right now contains enough fresh water to supply metropolitan Washington for 5,000 years or the drought striken state of California for 1,000 years.

The idea of towing icebergs to arid regions of the earth has been seriously discussed by scientists, but Nelson said he wouldn't think of trying it with this iceberg.

"I wouldn't even want to write the environmental impact statement," Nelson said. "Where would you park [TEXT OMITTED FROM SOURCE]