Scientists are studying sonar images of a shipwreck loaded with 3.5 million gallons of crude oil in the holds of a tanker that lies 4 miles off the scenic Central California coast like a rusting time bomb.

The American tanker Montebello was torpedoed by a Japanese submarine Dec. 23, 1941, only 16 days after the attack on Pearl Harbor, and sank in 900 feet of water. The Montebello has lain on the bottom ever since.

Japanese submarine attacks on ships off California's coast caused a near panic in those dark days of World War II when the West Coast was thought to be nearly undefended.

The surprise today is that a wartime shipwreck can still cause concern, nearly 70 years later.

Earlier this month, a robot submarine from the Monterey Bay Aquarium Research Institute dove down to the wreck to find answers to two major questions: Is the oil still aboard the Montebello, and if it is, will the oil eventually seep to the surface and cause a major environmental threat?

"We aren't taking any chances," said Stephen Edinger of the California Department of Fish and Game. "We are taking proactive steps to determine if there is a pollution threat, and if so, to prevent an oil release that could impact California's coastal areas."

The ship went down near San Simeon and the town of Cambria in a beautiful ocean region rich in sea life.

Though the site of the wreck is well known and has been explored before, this month's expedition using a submersible called an autonomous underwater vehicle was designed to produce new information about the shipwreck and its cargo.

Three different kinds of sonar on the submersible created three-dimensional images of the wrecked ship and the sea floor. The data it brought back are being analyzed by scientists onboard the institute's research vessel Zephyr and at an onshore lab. It is a slow and careful process and will require more dives.

The wartime drama began two days before Christmas 1941. The Montebello, a 440-foot-long Union Oil Co. tanker, was about to sail for Vancouver, British Columbia, from Port San Luis, a small Central Coast seaport. The vessel had finished loading 73,571 barrels (about 3 million gallons) of Santa Maria crude oil. The fuel tanks held about 104,034 gallons of fuel oil.

The officers and crew were worried about Japanese submarines just offshore. Several American-flagged ships had been attacked, one only 20 miles from Santa Cruz; at least two ships had been sunk. Things were so dicey that the skipper of the Montebello refused to take the ship to sea. He quit and was replaced by chief mate Olaf Eckstrom.

The Montebello sailed at midnight, heading north, making about 8 knots.

"She was one of the slowest ships on the coast," Dick Quincy said Monday. He is 91 now and lives in Danville, but then he was 22, an ordinary seaman - "the lowest job on the ship," he said.

About 5:45 a.m., he was on lookout on the bridge when he saw a submarine surface. "I called the mate and pointed it out," he said.

The Japanese submarine I-21 fired two torpedoes. One was a dud. The second hit near the bow. Eckstrom ordered the crew into the lifeboats. The Montebello was unarmed, and the 38-man ship's crew watched as the submarine fired its deck gun at the sinking ship, the rounds going right over the lifeboats, the men ducking.

"You held your breath," Quincy said. "It was a wild time."

The ship sank at 6:30. "She stood up on her bow and slid under," Eckstrom told The Chronicle in a story published the next day.

All four lifeboats made it safely; no one was killed or seriously hurt, though the men were badly shaken. They were taken to a nearby Army base, fed, questioned and told to keep quiet.

"We were told not to talk to anyone," Quincy said. Meanwhile, there was near panic: The Japanese were victorious everywhere in the Pacific. In San Francisco, there were air raid alerts and reports of battles at sea just off the coast.

"We heard people headed for the hills," Quincy said, "We heard all kinds of things."

A Japanese sub shelled an oil refinery near Santa Barbara in early 1942, but the Imperial Navy never attacked so close to California again.

Local fishermen have long known about the wreck of the Montebello, which is like a reef, teeming with fish.

In 1996, an underwater craft named Delta with two men aboard dove more than 800 feet toward the ship. The pair saw "a great grey wall looming out of the darkness," Jack Hunter, an underwater archaeologist for Caltrans, wrote in a report of the dive.

The Montebello was sitting upright on the bottom, "as if it were tied to a dock," Hunter said Monday.

He has looked at the sonar images from this month's expedition. He thinks the Montebello's cargo of oil is still aboard and has concerns that the wreck has deteriorated since he got a look 14 years ago.

"The structures of the ship are still intact," he said, "but they could collapse at some point."

The next move, scientists say, is to determine the condition of the Montebello's sunken oil. Crude oil similar to its cargo can congeal at low temperatures to a thick fluid, almost like sludge.

"The question is, if the oil got out of the ship, would it rise to the surface or would it stay (near the bottom) and drift away? No one is willing to say," Hunter said. "We have to assess what kind of risk it poses. And we have to offer an assessment as to how much time is left."

Exploration of the wreck has been suspended for the summer and will resume next year.