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A HISTORIC second world war German submarine lying under more than a kilometre of water off the US coast is being destroyed, as a result of oil released from the 2010 Deepwater Horizon spill.

The U-boat was sunk by an anti-submarine explosive in 1942 after a fierce battle. Now, it is being eaten away because seabed bacteria have an unexpected food source: the giant oil plume that entered the Gulf of Mexico nearly a decade ago.

Photos of wrecked Nazi sub U-166 taken before and after the聽Deepwater Horizon accident show聽that large holes have opened聽up in the hull and deck.

鈥淭he metal loss has accelerated聽after the spill and that鈥檚 very unusual,鈥� says Leila Hamdan at the University of Southern Mississippi.

The corrosion of metal in the ocean usually peaks in the early years of submersion, then slows. But analysis of the underwater photos show five times more of U-166 crumbled away in the four years including and after the spill than during the six years before.

The wreck was only discovered聽in 2001 during seabed聽surveys for oil firms including BP, which operated the聽Deepwater Horizon rig.

Hamdan and her colleagues placed metal discs on the seabed near the Deepwater Horizon spill to investigate the fast corrosion.

After 16聽weeks, the discs were covered with a slimy film and they were heavily corroded. They聽lost three times as much metal as identical discs placed 80聽kilometres away.

Genetic analysis revealed many聽different types of bacteria present in the film, including some known to feed on carbon sources in crude oil.

The team believes that these bacteria essentially consume the聽oil, and then their waste products corrode the metal.

鈥淲e can鈥檛 say for sure what is in聽the sediment around the submarine,鈥� says team member Jennifer Salerno at George Mason University in Virginia. 鈥淏ut we do know the change was down to the oil.鈥� The wrecked vessel lies less than 20 kilometres from the wellhead that blew during the Deepwater Horizon accident.

Oceanographers haven鈥檛 examined the U-boat since the most recent photos were taken in聽2013 and Hamdan is keen to go back and see what condition the wreck is in now. The earliest she could do so is next year聽鈥� and she says it is unlikely that the metal corrosion would have stopped.

鈥淕iven the historical and cultural significance of the U-166, we should go back,鈥� she says. 鈥淭he聽deep sea is a place that not a聽lot of us can connect with and this gives us a reason to care.鈥�

The photos of U-166 have already cleared up a wartime mystery over who sent it to the聽bottom. After it torpedoed and sunk the US passenger ship Robert E. Lee, the U-boat was attacked by a US ship captained by聽Herbert Claudius.

The US Navy didn鈥檛 believe his claims to have sunk it, stripped him of command and sent him back to military school to learn better tactics.

Images of the wreck showed that Claudius was right: one of his depth charges probably exploded on the submarine鈥檚 deck. In 2014, the navy apologised and awarded him a posthumous medal.