But such publicized events account for only a small part of the total amount of oil pollution in the oceans—and many of the other sources, such as automobile oil, go largely unnoticed, scientists say.

In fact, of the tens of millions of gallons of oil that enter North American oceans each year due to human activities, only 8 percent comes from tanker or oil pipeline spills, according to the 2003 book Oil in the Sea III by the U.S. National Research Council of the National Academy of Sciences, which is still considered the authority on oil-spill data.

Most oil pollution is "different than the pictures you see of beaches covered with tar and ducks getting stuck in it," said David Valentine, a biogeochemist at the University of California, Santa Barbara. (Read more about how pollution harms the oceans.)

Here are three little-reported sources of oil that contribute to oil pollution in North American oceans.

1. Natural Seeps

Natural seeps of oil underneath the Earth's surface account for 60 percent of the estimated total load in North American waters and 40 percent worldwide, according to the National Academy of Sciences.

These leakages occur when oil—which is lighter than water—escapes into the water column from highly pressurized seafloor rock. (Read about Gulf of Mexico seeps.)

Off Santa Barbara, California, some 20 to 25 tons of oil flows from seafloor cracks daily—making it one of the world's largest seeps.

Valentine, who studies the Santa Barbara seep, noted that much of the natural oil is consumed by ocean bacteria that have evolved to eat certain oil molecules. (Read about how nature tackles oil spills.)

But in "places which don't have natural oil seeps and you come along with an oil spill or a sewer pipe that delivers [oil pollution], organisms have not had an opportunity to adapt and are going to respond differently," said John Farrington, dean emeritus and marine geochemist with Woods Hole Oceanographic Institution in Massachusetts.

2. Cars and Other Land Vehicles

A "pretty big issue," Valentine said, is the oil on roads and other surfaces that's flushed into the sea during rainstorms.

Most cars drip oil onto the ground, usually on impermeable concrete or asphalt, and that oil ends up trickling into the ocean. In drier places like California, the oil builds up on the asphalt and, when it finally rains, the water shuttles large amounts of oil into the ocean.

"We're doing a much better job than 40 or 50 years ago of recycling motor oil," Woods Hole's Farrington said. "You can find storm sewers around the nation that have stencils on them that say 'don't dump, it goes to the sea.' So there's less input in that regard."

But he notes that there are still a lot of cars and trucks contributing to the "dribble, dribble, dribble" effect of slow leaks that end up on asphalt and contribute to runoff pollution.

Not surprisingly, this sort of invisible pollution is more subtle than the Galveston Bay spill, which is much more localized and visible, Valentine noted.

Oil runoff from land is "complex in that it can hang around [in the ocean] and move between water and sediment, [which] makes it difficult to effectively track."

A hotly debated topic, he added, is what these constant pulses of oil are doing to the environment and its inhabitants. Scientists know that animals directly exposed to oil suffer health problems, but what's unknown is the impact of low, chronic oil exposures on wildlife, he said. (Related: "On 25th Exxon Valdez Anniversary, Oil Still Clings to Beaches.")

3. Recreational Boats

People operating recreational craft, such as Jet Skis and boats, sometimes spill oil into the ocean.

"It's usually operational error, human error or unpreparedness, [or] lack of education. A lot of time mostly it's just negligence," said Aaron Barnett, a boating program specialist at Washington Sea Grant, a state-federal partnership aimed at marine research and outreach across Washington State.

"It's just not on [boaters'] radar scope. They're there to have fun, it's leisure, it's recreation. ... That means that certain things don't get dealt with, like proper engine maintenance."

Barnett added that boat owners will top off their fuel tanks as they would a car, and on a hot day the fuel expands and escapes through a vent.

Just like land-based pollution, though, oil spills by recreational boats are "hard to track, because about 80 percent of oil spills go unreported, so there's really no way to know" on what scale this is happening, Barnett said.