Torpedoes.

This is coolbert:

Submarines sunk by their own torpedo? Seems strange? NOT such an uncommon occurrence however. During a time of war and peace both.

Thanks to the Daily Mail this article describing the phenomenon as has existed ever since the earliest days of submarine warfare.

"Mystery of how H.L. Hunley's crew died is solved after 150 years: Men on board the American Civil War submarine were killed by the pressure blast of their OWN torpedo"

    * "The HL Hunley submarine fought for the confederacy and was sunk in 1864"
    * "It was raised from the bottom of the ocean in 2000 with its eight members of crew still aboard"
    * "Experts studying the submarine believe a condition called 'blast lung' was responsible for their deaths"
    * "The force of the torpedo's explosion would have traveled through soft tissue in their bodies"
    * "They calculate the chance of fatal lung trauma to be at least 85 per cent"

Again, submarines sunk by their own torpedoes not such an absolutely rare event. Historical instances to include but probably not limited to:

1. American USS Tang  [WW2].
2. Russian APL Kursk.
3. German U-972. U-377 [WW2]

Quite possibly also but never confirmed:

4. American USS Scorpion.

See in addition an Internet web site listing those instances during the Second World War of American submarines nearly sunk by their own "circular-running" torpedoes.

P.S.: Further two more submarines as sunk by their own torpedoes. 1. USS Tullibee. 2. HMS Sidon.

coolbert.