Oil from the Exxon Valdez spill in Alaska 17 years ago could still be threatening wildlife, according to scientists who surveyed the shoreline.

Researchers at the National Marine Fisheries Service in Juneau, Alaska, found oil buried in the sand and silt that only dries during the lowest tides. The area is a prime feeding ground for sea otters, ducks and other wildlife.

Scientists had thought that beaches were most contaminated by oil around their higher tide levels.

However, now they've found buried oil that isn't exposed to weathering and other elements that might help it degrade, said research chemist Jeffrey Short, one of the authors of the study.

Short and his colleagues randomly dug 662 pits along the shoreline of northern Knight Island, one of the earliest and worst affected areas of the Exxon Valdez spill on March 24, 1989.

Sea otters dig pits at high tide when the zone is immersed, and then dive down, disturbing the sediments to look for clams.

If the mammals run into spilled oil, it's likely to get into their fur and then be ingested when they groom themselves, Short said.

Scientists don't know if ingested oil is toxic to mammals, but say the contaminants could hamper the recovery of sea otter populations in the worst affected areas.

The study appears in the May 16 online issue of the American Chemical Society’s journal, Environmental Science & Technology.

ExxonMobil disagrees with the conclusions, a spokesman said, noting the company's research suggests the area has recovered and is healthy.