Chemical analyses find that this 25-year-old oil is from the Exxon Valdez spill, when the tanker ran aground on Bligh Reef in Prince William Sound (map) in 1989. And to the surprise of the scientists, the oil still has most of the same chemical compounds as oil sampled 11 days after the initial spill. (See "Exxon Valdez Anniversary: 20 Years Later, Oil Remains.")

The oil's presence in areas that were cleaned right after the spill 25 years ago points to the need to monitor certain environments long after the visible effects disappear, the researchers say.

There are two main reasons why there's still oil on some of the beaches of the Kenai Fjords and Katmai National Parks and Preserves in