The Battle of Jutland, between the British and the Germans in June 1916, was punctuated by exploding British battle cruisers. Three of them blew up and disappeared, suddenly. In each case almost the entire crew, in each case over 1,000 men, were lost.
The Indefatigable, last in a line of battle cruisers, was first to blow up. Robert Massie tells what happened:
"In New Zealand just ahead, the navigating officer looked back at Indefatigable.
The British ships blew up because crews had modified safety devices meant to keep fire from hits on turrets from exploding the powder magazines. Devices that might have protected the magazines had been removed to allow ships to fire faster and win gunnery competitions. Massie explains:
"...There is no evidence that Indefatigable, Queen Mary, and Invincible blew up because German 11-inch or 12-inch shells penetrated their armored hulls and burst inside their magazines. Rather, the almost certain cause of these cataclysmic explosions was that the turret systems of British battle cruisers lacked adequate flashtight arrangements and that, in each of these ships, a shell bursting inside the upper turret had ignited powder waiting to be loaded into the guns, sending a bolt of flame flashing unimpeded down the sixty-foot hoist into the powder magazines. Assuming this to be true, blame lay not with the design of British ships but with the deliberate decision by captains and gunnery officers to discard the flashproof scuttles originally built into British dreadnaughts. The Royal Navy made a cult of gunnery. To win peacetime gunnery competitions, gun crews were encouraged to fire as rapidly as possible. Quick loading and firing required a constant supply of ammunition at the breech of the gun, and thus a continuous flow of powder bags moving out of the magazines and up the hoists to the guns. Safety became secondary; gunnery officers began leaving magazine doors and scuttles open to facilitate movement; eventually, in some ships, these cumbersome barriers were removed. But for this weakness none of the three battle cruisers might have been lost."
Source: Robert K. Massie. Castles of Steel. Britain, Germany, and the Winning of the Great War at Sea. Random House. 2003. The destruction of Indefatigable is described on page 593. The importance of gunnery competition and the modifications made to the turret structures is described on page 667.
We were altering course to port at the time and it seemed as if her steering was damaged as she did not follow around in our wake but held on until she was about five hundred yards on our starboard quarter. While we were still looking at her, she was hit again by two shells, one on the forecastle and one on the fore turret. Both shells appeared to explode on impact. There was an interval of about thirty seconds and then the ship completely blew up. The main explosion started with sheets of flame, followed immediately by a dense dark smoke cloud which obscured the ship from view. All sorts of stuff was blown into the air, a fifty foot packet boat being blown up about two hundred feet, apparently intact though upside down.
Stricken, with smoke pouring from her shattered hull, Indefatigable rolled slowly onto her side, all the while driving through the water. Then the huge vessel turned completely over and plunged, taking with her 1,017 officers and men. Only two seamen survived..."