Until now, marine biologists thought the deepest-diving air-breathing animals were sperm whales, which have been recorded at 3,740 feet below the ocean's surface.

New findings transfer the title to the leatherback sea turtle, recently tracked to at least 3,936 feet, almost three-quarters of a mile under the waves.

Leatherbacks, which can weigh more than 1,300 pounds, are the largest living turtles and the most widely distributed reptile in the world. They ply both coasts of the Atlantic from Newfoundland to Argentina and Norway to South Africa and the Pacific coast from the Soviet Union to Tasmania, south of Australia.

Because leatherbacks maintain warm body temperatures even in cold waters, some scientists speculate that the reptiles may have active temperature-maintaining mechanisms comparable to those of warm-blooded mammals. Others say it may be their bulk -- a high volume within a small surface area -- that keeps them from losing too much heat.

The turtle's diving depth was monitored by Scott Eckert of the University of Georgia with the aid of a pressure-sensitive recording device fastened to the animal, which Eckert read each time the turtle surfaced for air. The device was calibrated only to 3,280 feet (1,000 meters), but on the record dive, the indicator went well off the scale. Eckert estimated the true depth -- on the basis of the time the same animal spent in shallower dives -- at a minimum of 3,936 feet (1,200 meters).