According to the New Scientist, rusting ships sunk in the Second World War may be about to release a huge amount of oil into oceans around the world. It cites estimates that 8569 sunken vessels still contain 2.5 to 20 million tonnes of oil. In comparison, the Deepwater Horizon is estimated to have spilt 1.1 million tonnes; the Exxon Valdez caused enormous and lasting damage with only 40,000 tonnes of oil.
When will the wrecks leak? According to the New Scientist, most ships sunk during the war were made of steel plate between 19 and 25 mm thick, which normally loses structural integrity once it has lost between 25 and 50% of its thickness, or 4.5 to 12 millimeters of steel. On average, steel corrodes 1 mm a decade, so by now the average ship would have lost 7 mm, well into the danger zone. Of course, the ships were probably also damaged when they were sunk, and several such wrecks have already caused significant contamination around the world. The New Scientist predicts many more such problems in the next few decades.
Who will pay for cleaning up oil from a 70-year-old wreck? The owner of the ship, who may be long gone, or the Armed Forces that sank it? Since no clear legal rules apply, most wrecks just keep rusting slowly away.