Alaska ? An engineering professor has figured out why oil remains trapped along miles of gravel beaches more than 20 years after the Exxon Valdez tanker disaster in Prince William Sound.

An estimated 20,000 gallons of crude remain in Prince William Sound, even though oil remaining after the nearly 11-million-gallon spill had been expected to biodegrade and wash away within a few years.

The problem: The gravelly beaches of Prince William Sound are trapping the oil between two layers of rock, with larger rocks on top and finer gravel underneath, according to Michel C. Boufadel, chairman of the Department of Civil and Environmental Engineering at Temple University. His study appeared Sunday in Nature Geoscience’s online publication.