Tar balls on beaches and oil-coated cormorants make the presence of oil in the cold, gray Pacific undeniable. Here are some of the top culprits:

• Oil platform leaks. In 1969 the blowout of a Union Oil well poured more than 3 million gallons into the Santa Barbara channel. The slick spread along 35 miles of coastline and contaminated roughly 800 square miles of ocean.

• Refinery leaks. In 1988 a tank at a Shell oil refinery in Contra Costa County drained 400,000 gallons of oil into a nearby creek, and from there into a salt marsh and Suisun Bay.

• Transportation spills. Accidents involving tanker trucks, trains and ships carrying oil are responsible for some of the biggest and most disastrous oil spills in California history. Last November the container ship Cosco Busan crashed into a bridge support and poured 58,000 gallons of fuel oil into San Francisco Bay.

• Oil pipeline breaks. The 1992 rupture of a Unocal pipe in San Luis Obispo County discharged more than 24,000 gallons of crude oil into the Pacific.

• Road runoff. Oil leaks from cars and small petroleum spills wash from roads into storm drains and then into the sea, adding up to major pollution. Officials estimate the cumulative impact of oil from road runoff is even greater than that from major spills.

• Sunken vessels. The Montebello may not have sprung a leak yet, but others have. The S.S. Jacob Luckenbach sank in the Gulf of the Farallones in 1953, loaded with 500,000 gallons of oil. It leaked sporadically from 1992 through 2003, when the U.S. Coast Guard removed much of the oil and sealed the rest inside the still-sunken ship.

• Natural oil seeps. A constant dribble of crude oil oozes from beneath the sea floor in places such as the Santa Barbara channel. The goop can affect wildlife and form tar balls that wash up on shore, but it’s generally not as toxic as oil that’s been processed into fuel. State officials take a hands-off approach to natural seeps, letting nature take its course instead.

Sources: California Department of Fish and Game Office of Spill Prevention and Response Natural Resource Damage Assessment and Restoration website, www.dfg.ca.gov/ospr/spill/nrda/nrda.html, and OSPR spokeswoman Carol Singleton.