Twenty years ago the Exxon Valdez tanker ran aground in Alaska, releasing 11 million gallons of crude oil into Prince William Sound..

Today you might think that the coast is clear and clean. But it's not quite clean, even after 20 years. Mandy Lindberg, a scientist with the National Oceanic and Atmospheric Administration (NOAA), demonstrates with a shovel of sand and broken rock, revealing a glistening pool of brackish oil. The crude can be chemically typed to the Exxon Valdez, and more oil can be found beneath the beach at Death Marsh, a site that was especially afflicted by the spill,  and at a number of islands around the Sound. "I wouldn't have possibly believed the oil would last this long," says Lindberg. "Studying the spill has been a great learning experience, but if we had known in the years after the spill what we know now, we would have been looking for oil much earlier."

What scientists like Lindberg know now is that the legacy of the Exxon Valdez is still visible — physically, on the beaches of Prince William Sound and in the animal populations in these sensitive waters that have yet to rebound fully. Scientists from NOAA have carried out major studies that show oil still remains just beneath the surface in many parts of the Sound — close enough for animals to be affected by it. "The oil may not leak out in quantities that are immediately visible, but that doesn't mean it's not there," says Jeep Rice, a NOAA scientist who has led the studies. "We thought the cleanup would be a one-shot deal — but it's still lingering."

Altogether, the NOAA scientists estimated that about 20,000 gallons of oil still remained around the Sound, usually buried between 5 in. and 1 ft. below the surface.  Those 20,000 gallons, out of at least 11 million spilled, might not seem like much, and scientists initially assumed that whatever oil was left behind during the original cleanup would eventually break down naturally.

Read further at TIME.