Pensacola BeachUniversity of South Florida coastal geologists (left to right) Stoddard Pickrel, Katie Brutsché, and Jun Cheng dig into a beach near Pensacola Beach, Florida (map), on Thursday. It didn't take long for the scientists to strike black gold.During a series of digs, oil patties and tarballs were found just beneath beaches dirtied by the Gulf of Mexico oil spill. The discoveries suggest that toxic oil lies hidden under even "clean" patches of beaches along the U.S. Gulf Coast—and that oil-spill cleanup crews are only scratching the surface.Because the buried oil is both harder to clean and slower to break down, it could be a long-lasting threat to beachgoers, both animal and human, experts say.(See "Oil Found in Gulf Beach Sand, Even After Cleanups.")—Adapted from a story by Christine Dell'Amore in Pensacola Beach, Florida, and Gulf Islands National Seashore

Photograph by Chris Combs, National Geographic