The West Loch Disaster was a maritime accident during World War II at the Pearl Harbor U.S. Naval Base in Hawaii. The incident, which occurred just after 1500 hrs. on Sunday 21 May 1944, began following an explosion in a staging area for Landing Ships, Tank (LSTs) and other amphibious assault ships in West Loch. A fire quickly spread among the ships being prepared for Operation Forager, the invasion of the Japanese-held Mariana Islands. Over the next 24 hours, six LSTs sank, 163 naval personnel died and 396 were injured.
A subsequent Naval Board of Inquiry never determined the exact cause of the disaster but concluded that the initial explosion was caused when a mortar round aboard LST-353 detonated during an unloading operation because it was either dropped or went off when gasoline vapors ignited. The incident – together with the Port Chicago disaster two months later – led to major changes in weapon handling practices within the United States Navy.
The LST wreckage was quickly cleared in a salvage operation and dumped at sea 3 mi (2.6 nmi; 4.8 km) south of Hawaii. Only the hull of the partially beached LST-480 was left in West Loch. A press blackout was enforced and naval personnel were ordered not to talk about the incident. The disaster was classified until 1960 and is therefore not well known.
During the salvage and removal of the wrecks from West Loch, the U.S. Navy found remains of a Japanese midget submarine. Researchers now believe this to be the fifth Japanese midget submarine used in the attack in December 1941.