ANACORTES, WASH., JAN. 31 -- A barge carrying 318,000 gallons of heavy oil sank four miles offshore and began leaking today, the Coast Guard said.

The barge, its cargo in 12 sealed compartments, began listing unexplainably in calm seas and went down in 136 feet of water, said Coast Guard spokesman Roy Compton.

Murky water and strong currents prevented divers from determining the extent of the leak, and a more sophisticated diving team was summoned, officials said.

"As long as there is a barge sunk out there with 9,000 barrels of oil product, there is the potential for a major problem," Compton said. Coast Guard Seaman Veronica Cady said divers from National Oceanic and Atmospheric Administration could go no deeper than 80 feet late this afternoon.

"When they came up, there was oil on their suits," she said. "It's like Karo syrup {in consistency} and it smells."

NOAA spokesman David Kennedy said disposable diapers wrapped around a lead ball and lowered into the water came up with a smattering of oil, indicating the oil was being carried away by the current instead of settling in a pool. Despite a search during the day, "we're not finding any soiled birds anywhere," he added.