The day it happened, biologist Jim Bodkin was watching sea otters off the coast of Big Sur, Calif. In Seward, Alaska, orca researcher Craig Matkin tinkered beneath the bow of his 42-foot purse seiner, Lucky Star. One hundred forty nautical miles to the east, Capt. Dave Janka stood in the kitchen of his family's remote cabin, hand-grinding coffee and listening to public radio on a Walkman. When the news came on, he nearly "dropped the whole thing." Then he jumped in his Zodiac and motored eight miles to one of the greatest environmental disasters of our time: the Exxon Valdez tanker foundering on a reef, hemorrhaging 11 million gallons of crude into Prince William Sound. It was March 24, 1989.

The early-morning scene was eerily calm, but word was spreading. As Janka idled his skiff, Matkin and thousands of other Alaskans hastily jammed gear into boats, cars and planes and swarmed toward the Sound, which had nowhere near enough equipment on hand to tackle the spill.

Worse, while Exxon mustered its resources, a winter storm blew in, driving crude against shore and into the Gulf of Alaska. Oil coated more than 1,000 miles of beaches, none with road access. Hundreds of thousands of sea birds and mammals washed up dead. Even after a cutting-edge "supersucker" was barged in from Prudhoe Bay, volunteers armed with bath towels and five-gallon buckets handled the bulk of the cleanup. "I was out for I don't know how many days straight," Matkin says. "The feeling of weightlessness was like being in outer space."

If the devastation had a silver lining, it was the billion-dollar settlement paid to the Exxon Valdez Oil Spill Trustee Council in 1991. The money bankrolls an ecosystem-monitoring program called Gulf Watch Alaska that includes Matkin, Bodkin and 21 other scientists. With 25 years of data and enough funding for 18 more, Gulf Watch is providing the first comprehensive, long-term look at a marine oil spill's consequences – knowledge that could guide response to other spills and, as oil exploration and shipping heats up in the Arctic, even help prevent them.

Scientists predicted Prince William Sound would recover within 15 years, but today, residual oil continues to harm both human and ecological communities. "It's very insidious," says Mark Carls, a toxicologist with the National Oceanic and Atmospheric Administration. "I think we were all surprised."

Thirteen of the 35 whales in the Sound's resident, fish-eating pod died soon after the spill and the group still hasn't recovered.

"You think about oil spills and you think of those photos of oiled otters and birds dying on beaches," adds Dan Esler, a wildlife biologist with the U.S. Geological Survey. "But chronic effects for some species can be of the same magnitude as the immediate effects."

Today, it's hard to imagine a Prince William Sound battered by environmental catastrophe. Green-shouldered mountains plummet to the sea. Kayakers paddle beneath glaciers. Bald eagles keep watch over rugged, rocky beaches.

If you know where to look, though, there's no shortage of oil. Officially, 21,000 gallons remain, but many consider the estimate conservative. And because much of it is trapped beneath rocks and cut off from oxygen, it has the same composition as oil that's only 11 days old. Even where oil is exposed to the elements, Carls says, the hydrocarbon molecules that persist longest also tend to be the most toxic. That might explain why some species – like Craig Matkin's orcas – are still reeling.

Matkin's fascination with orcas began in 1976, when he first encountered them as a young salmon biologist. By the time of the spill, he could recognize individual whales on sight. They were like family – which made seeing them slicked in oil all the more painful.

Thirteen of the 35 whales in the Sound's resident, fish-eating pod died soon after the spill and the group still hasn't recovered, even as other Alaskan orcas are thriving. Worse off are the genetically distinct "Chugach transients," which pass through Prince William Sound as they roam the coast hunting seals and sea lions. The pod had 22 members before the spill; today, there are only seven. Because they're at the top of the food chain, the Chugach transients accumulate contaminants from everything below them. High levels of PCBs and DDT in their blubber may be causing reproductive problems: The Chugach transients haven't birthed a single surviving calf since 1984.