On 19 April 1989, an explosion occurred in turret 2 of the battleship USS Iowa (BB 61) as the ship conducted gunnery practice near Puerto Rico. The explosion, which began in the gunpowder charge of the center gun, spread through the three gun rooms and much of the lower levels of the turret. Forty-seven Sailors died.
The problem of storing and handling large quantities of high explosives in a shipboard environment, whether in peactime or in combat, posed a major challenge for ship and ordnance designers from the earliest enclosed mounts of the late 19th century.
The possibility that a fire or explosion anywhere in the turret could spread to the magazines below and sink the ship resulted in continuing refinements to armor (to keep out enemy shellfire), complicated interlocks on powder and shell handling equipment (to prevent fire from having an uninterrupted path to the magazine), and crew procedures (to ensure that the mechanical systems would have a chance to work).
Turret explosions on Navy ships, while rare, caused significant loss of life, but internal protective systems prevented the magazine explosions that would have destroyed the ship. USS Mississippi (BB 41) had the misfortune to experience two: a peacetime explosion in 1924 that took 48 lives, and an explosion during a 1943 wartime shore bombardment mission that killed 43. Twenty sailors died and 36 were injured in a 1972 explosion in turret 2 of the cruiser USS Newport News (CA 148), operating off Vietnam. These tragedies, and the Iowa explosion, are a reminder that naval combatants and their crews handle dangerous things in dangerous situations for a living, and that the highest standards of crew training and ship design are nonnegotiable.
RIP Shipmates. You stand Relieved. We have the watch.