The oil spills were a mystery. Dozens throughout the 1990s had killed more than 50,000 seabirds and several sea otters off the California coast.
"We couldn't find a source," said Lisa Symons, Damage Assessment and Resource Protection Coordinator at the National Oceanic and Atmospheric Association. Analyzing the oil, investigators determined it all came from the same source and was not a natural underwater seep of California crude.
Finally, in 2002, Symon's team pinpointed the culprit: the wreck of the SS Jacob Luckenbach, a freighter headed to Korea laden with supplies for the Korean War effort.
The Luckenbach sank off the California coast in the summer of 1953 after colliding with another ship. After decades on the ocean floor, the ship began leaking oil through corroded ventilation pipes, belching up its stores a bit at a time, especially as storms rocked the wreck.
But the Luckenbach is not an isolated threat. Around the world, governments are taking stock of the oil submerged aboard long-sunk vessels.
Over 8,500 potentially polluting shipwrecks sit on the bottom of the sea and more than 6,300 of these are from the World War II era. Now, after 70-odd years of corrosion and battering by currents, some may not hold their toxic cargo for much longer.
"A lot of these wrecks are reaching a point in their decay curve where they may experience some structural changes and they may leak some pollution," said David Conlin, chief of the National Park Service's Submerged Resources Center, headquartered in Lakewood, Colo.
"This is a global problem," he added. "Word War II really was the first war that was based on petroleum. This was the first war where ships carrying petroleum were the first targets for attack by the enemy, and because it was a global war, these ships are spread out all over the place."
The amount of oil contained in these ships could be anywhere from 757 million to 6 billion gallons according to a 2005 assessment prepared by Dagmar Ektin of Environmental Research Consulting in Cortlandt Manor, NY and others.
That amount is between 3.6 and 30 times NOAA's estimate for the amount of oil spilled by the Deepwater Horizon disaster, 205.8 million gallons. The Exxon Valdez spilled 11 million gallons. However, the submerged ship's threat is different from either of these, since the boats are scattered and won't come apart all at once.
In an attempt to be proactive in addressing the growing threat that these old ships pose, Symon's team at NOAA is undertaking an inventory of ships in U.S. waters, gathering all the information they can to produce a prioritized list of the ships that pose the greatest risk.
They have gone back to old records of the ships and their cargo, and have combined that information with recreational or technical divers' reports about the wrecks, when possible. They try to determine whether the Navy blew the wrecks up to eliminate a navigation hazard, for instance, or other factors that would influence how much oil was on board.
Then, for relevant cases, team members use models to forecast where a spill would go using models of currents and winds, and they assess the potential economic impacts of a given spill, Symons said.
Ships corrode faster in highly oxygenated or in warmer water, but warmer water also promotes the growth of marine organisms over the hulls, which can form a "scab" on the hulls preserving them, according to Conlin.
The majority of wrecks are along the Atlantic coast where German U-boats did the most damage. Symons said her teams have narrowed the list down to about 40-45 of the highest priority vessels and they are continuing to narrow the list, though information in many cases is spotty.
"These operations are tremendously expensive," Conlin noted, "But the other thing to bear in mind is that it's far less expensive to clean up a shipwreck prior to the release of oil rather than after the oil is released."
Cleanup of the Luckenback cost $18-22 million, Symons said. Several things drove up the expense: escaped oil had already causing damages requiring remediation, the wreck sat far offshore, and the oil turned out to be the consistency of peanut butter, so salvage crews had to heat it up to pump it out.
In general, cleaning up the wrecks involves tapping the compartments and pumping out the oil, but it can be difficult to know which compartments hold the oil after so many decades, and depending on the orientation of the wreck, it can be hard to get at them, Symons and Conlin noted.
Conlin was part of the team analyzing the USS Arizona, the battleship sunk in the 1941 Pearl Harbor attack. Although it harbors some 600,000 gallons of fuel oil, it is stable for now, Conlin said.
As a battleship, "it's designed to absorb damage," Conlin said, with thick metal hull plates and many small fuel tanks.
"Based on the corrosion rate, based on the amount of hull material that still remains and based on our analysis of the structural forces at work, we do not anticipate significant structural collapse for hundreds of years," he said.
The USS Arizona, like others of the wrecks NOAA is considering, is a war grave, which means the wrecks will be left undisturbed if possible, and treated with extra measures of respect.
The problems are not limited to oil, researchers noted. Some ships carry munitions, chemical wastes, radioactive materials and more.
"It's an issue that has escaped people's attention for a long time. When things are underwater, they are out of sight and out of mind. It's important to develop a plan to engage with this," Conlin said.