2012 marked the 70th anniversary of a series of World War II battles in the Pacific Ocean and on its islands, which are collectively known as the “Pacific theatre.” While the battles are long over, thousands of wrecked boats and planes from many nations still rest on the seafloor. These wreck sites represent a twin legacy: one a memorial gravesite and historical marker, and the other a potential source of pollution from the wrecks into the sea.

We call the first “Underwater Cultural Heritage”—the wrecks that are part of our collective planetary history. You can find U.S. wrecks from World War II scattered around the Pacific, the Atlantic, and even Lake Michigan, where aircraft carrier landing practice and training took place. Although the precise locations remain unknown, it’s estimated that there are some 20,000 wreck sites in U.S. waters alone. So how do we decide what wrecks to salvage or recover, a process that can take millions of dollars per location?

While historical importance plays a role, the potential for pollution sometimes takes precedence. Many airplanes and ships break apart when they sink, which assures that whatever fuel and other material they were carrying leaked out and washed away long ago. However, others sank with their fuel, munitions and other cargo intact, and they can become the source of significant localized oil spills over time. To avoid this, the best preemptive solution is to pump out the fuel before the tanks fail.

The U.S. National Oceanic and Atmospheric Administration has begun to address the 106 highest priority wrecks: a shortlist culled from thousands of wrecks in U.S. waters. First, NOAA’s Office of National Marine Sanctuaries eliminated anything less than 125 feet long because the fuel load is less threatening to sea life. Then they considered the stories of how ships and planes sank to determine whether they were likely to have fuel onboard. And then on through a series of additional screens such as age, hull material, vessel type and proximity to natural resources, they arrived at the highest priority wrecks.

On top of this list, workers have to clean up emergency leaks from wrecks. For example, after the sheen of a series of mystery spills appeared on the ocean surface off the California coast in 2002, the US Coast Guard and NOAA’s Office of Response and Restoration discovered leaking fuel tanks aboard the sunken SS Jacob Luckenbach below. The Coast Guard and Oil Spill Liability Trust Fund removed much of the oil and sealed the rest before its many gallons could leak into the nearby waters where fish and other animals feed and live.

But it’s not just U.S. wrecks that threaten the ocean—it’s also wrecks from around the world. International law holds that any vessel belonging to a government continues to belong to that government wherever it crashes or sinks. Some governments will claim their sunken vessels, often asking for the site to be protected as sacrosanct.

However, despite the sanctity of a site, hazardous materials may still lie within sunken military or merchant vessels. In this case, who is potentially responsible for preventing harm to environmental health, and who is liable in the event of such harm? In the U.S., the Navy has taken a look at these problems on a case-by-case basis. For World War II vessels in particular, several groups, individuals and governments are working together to look at the artifacts and potential threats that might reside in the vast array of underwater sites.