Was it a Mine? An RPG? A Submarine Collision?

More and more, it sounds as if the authorities believe the damage to the Japanese tanker was an external hostile act, though its exact nature is unclear.

Lloyd's List, the shippintg daily, is reporting it may have been a grenade; I'd assume they mean a rocket-propelled grenade, and a more detailed article is apparently behind a subscription wall; some other reports have said crewmen saw a flash of light before the explosion, which would imply some kind of rocket or missile. Somali pirates have used RPGs to seize ships. On the other hznd, one expert quoted in The National leans towards a mine, on the grounds RPG damage would be smaller and round. (A mine should have done more damage, but the article suggests it could be a 20-year-old mine from the tanker wars era of the late 1980s. But that wouldn't explain a flash of light beforehand.)

As this New York Times article notes, there is a history of submarine collisions in the shallow and narrow waters of the Gulf, with a US sub colliding with a Japanese tanker in 2007, and in 2009 another US sub collided with a US Navy surface vessel. But this time the US Navy says none of its subs were involved.

British and French subs often deploy to the Gulf, and Iran has three Kilo-class subs; Israel publicly sent one of its subs through the Suez Canal last year; published reports have suggested it might keep one sub in the region, at least in high tension periods.