He said there was speculation that the vessel could be a mini or midget submarine from the war.

BBC: Experts to examine River Foyle, Derry, wreck

The documentary is hosted by Tom Brokaw and features Ballard's November 2000 underwater search for a lost Japanese midget submarine whose location is one of the enduring mysteries of the battle.

FORBES: Pursuits | Found At Sea

That is perhaps the best explanation why a midget North Korean submarine torpedoed the Cheonan, killing 46 South Korean sailors this March.

FORBES: Magazine Article