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The sunken remains of thousands of ships litter the Pacific Ocean’s depths—relics of the Second World War. Far from benign markers of history, these wrecks pose a pressing problem: every day brings them closer to spilling the oil they still carry.

Chuuk Lagoon, a natural harbor in the Federated States of Micronesia, contains a particularly high concentration of these potential polluters. During the war, Japan used the site as a naval base, but in 1944, Allied forces bombed the lagoon leaving hundreds of wrecks scattered across an area about the size of the Hawaiian island of Maui.

After rusting away for 75 years, Chuuk Lagoon’s wrecks are wearing thin. “We can actually see oil seeping from some of the shipwrecks,” says Ian MacLeod, a now-retired shipwreck conservator from the Western Australian Museum. Researchers and divers frequently report black pearls of oil bubbling up from the vessels, creating greasy slicks at the water’s surface.

From 2002 to 2015, MacLeod led corrosion studies on the wrecks. Many were nearing collapse. “The situation is likely to become critical in the next four or five years,” he says. A typhoon could break open the ships’ brittle fuel tanks, unleashing much larger volumes of oil. Chuuk Lagoon is an example of a global problem—around the world, more than 8,000 shipwrecks threaten to release their remaining oil.

A spill from one of these shipwrecks could have devastating effects on Pacific island nations’ economies, which rely heavily on tourism and fisheries, MacLeod says. Oil released from wrecks can damage coral reefs, wipe out marine life, and clog mangrove forests, which serve as breeding grounds for fish. “The whole commercial livelihood of the island community would be destroyed,” MacLeod says.

There’s precedent for such a disaster. In 2001, for example, a sunken US oil tanker spilled more than 68,000 liters of oil off the coast of Yap State in Micronesia, coating nearby beaches in black sludge and prompting government officials to temporarily ban fishing in the area.

The threat of future spills is motivating researchers and activists. The Japan Mine Action Service, an organization of military veteran volunteers, sends divers to Chuuk Lagoon to clean up oil pooling inside Japanese wrecks using absorptive rags and handpumps. Other groups are taking a more preemptive approach. The Major Projects Foundation, an Australian nonprofit, put together a team of scientists, engineers, and archaeologists to assess which wrecks in the Pacific pose the greatest pollution risk.

Experts still don’t know which wrecks contain oil, or how much they hold, says Paul Adams, the director of the Major Projects Foundation. Current estimates suggest that South Pacific wrecks are sitting on anywhere from 500 million to 4.5 billion liters of oil—at least 12 times more than was spilled by the Exxon Valdez in 1989.

To refine their understanding of how much oil might still be in each of these thousands of ships, Adams and his colleagues have been scouring naval and historical records, gathering accounts of what kind of damage each ship incurred before it sank, how far it traveled since it last refueled, and what it was carrying. The team is comparing these records against information about the wreck’s current location, level of corrosion, and the potential impact it leaking would have on reefs, mangroves, and communities. So far, they have filtered a list of 3,000 ships down to about 50 high-risk wrecks. Funding permitting, they hope to survey these wrecks in dives next year.

But understanding which ships pose the biggest threat isn’t enough to solve the problem. For that, people need to actually get down there and drain the oil from the ships. One major hurdle stands in the way, says geochemist Jacqueline Michel: “There’s no money.”

Removing oil from a single wreck costs millions of dollars, and Pacific island governments likely won’t be able to shoulder the cost. Most of the wrecks in the Pacific belong to the United States and Japan, but international regulations surrounding shipwreck cleanup are weak, Michel says. “Normally people don’t pay attention to them until they start leaking.”

Ignoring the problem won’t make it go away. “Time is really critical,” Adams says. If the work doesn’t start now, it will be too late.