In May 1944, Pearl Harbor was the staging ground for the voyage to Saipan. In the West Loch of the harbor, there were a large number of landing ships full of unsecured depots of fuel and ammunition docked alongside each other. On the day of the incident, one of the ships suddenly exploded and sent crew and debris flying in multiple directions (via Naval History Blog). As emergency personnel and anyone nearby tried to help whomever they could, the debris set off a chain-reaction and other ships began to explode as well. 

After enormous efforts to contain the disaster (both physically and in the media), 163 were found to have been killed while 396 were injured (via San Diego Union-Tribune). Despite being called “The Second Pearl Harbor” by witnesses, this was not the result of an enemy attack. The truth is instead shrouded in mystery, as all crew members near the estimated blast origin were killed. Per the recollections of survivors, lit cigarettes were a common sight at the time and one could have very easily been the catalyst. While the specific cause may never be known, it was almost certainly facilitated by the handling of war materials and led to major safety reforms in the following months (via Naval History and Heritage).