On 9th of July 1917 HMS Vanguard exploded in Scapa Flow (the result of an internal explosion of faulty cordite), killing 804 men.

HMS Vanguard was one of three St Vincent-class dreadnought battleships built for the Royal Navy in the first decade of the 20th century. She spent her career assigned to the Home and Grand Fleets. Aside from participating in the Battle of Jutland in May 1916 and the inconclusive action of 19 August several months later, her service during World War I mostly consisted of routine patrols and training in the North Sea.Shortly before midnight on 9 July 1917 at Scapa Flow, Vanguard suffered a series of magazine explosions.

She sank almost instantly, killing 843 of the 845 men aboard. The wreck was heavily salvaged after the war, but was eventually protected as a war grave in 1984. It was designated as a controlled site under the Protection of Military Remains Act 1986, and diving on the wreck is generally forbidden.

Wikipedia

A definitive reason for the cause of the cordite explosion has never been found. The possibilities:

• spontaneous detonation of cordite, which had become unstable
• the cordite having caught fire from heating in an adjacent compartment
• sabotage

Of the three, sabotage is the least likely: no agency or individual has ever claimed responsibility; there has never been any evidence turn up in support of the theory; and just as important is the fact that when she was lost, Vanguard was one of the least modern ships in the Grand Fleet. The security measures for her were no different than for the more recent arrivals in Scapa Flow. It stands to reason that any ‘agent’ with the ability to destroy a Royal Navy capital ship would choose one of the more powerful ones. Instead, the most likely cause was the second: a fire in an adjacent compartment (coal bunker or patent fuel space) which smouldered away undetected, long enough for some cordite near the adjoining bulkhead to overheat to dangerous levels.

Loss of HMS Vanguard

In the local newspaper , The Orkney Herald and Advertiser the following is reported on 18th July 1917:

A DREADNOUGHT BLOWN UP

INTERNAL EXPLOSION IN THE VANGUARD

ALL BUT TWO ON BOARD KILLED

Admiralty Friday – The Secretary of the Admiralty regrets to report that H.M.S. Vanguard (Captain James D. Dick R.N.) blew up while at anchor of the night of the 9th inst. , as the result of an internal explosion. The ship sank immediately, and there were only three survivors among those who were on board the ship at the time of the disaster. – viz. one officer and two men. The officer has since died. There were, however, twenty four officers and seventy one men not on board the ship at the time, thus bringing the total number of survivors up to ninety seven. All the next of kin have been informed. A full inquiry has been ordered.

The report continues:

FULL CREW 870

FOURTH BRITISH LOSS OF THE KIND IN THE WAR

….She is the fourth British ship lost at anchor during the war. The others were:-

BULWARK, Battleship at Sheerness, November 26th 1914, between 700 and 800 lost

PRINCESS IRENE , (Auxiliary ship ) May 27th 1915, 356 crew, 75 workmen lost

NATAL, Armoured cruiser, December 30th 1915, 428 lost

Of the three who initially survived the explosion, Lieutenant Commander ACH Duke, later died of his injuries.

“The other two men, Marine J Williams and Stoker 1st Class FW Cox, miraculously survived the explosion. The men are said to have been unable to recall the disaster and described being asleep in their bunks before waking up swimming away from the battleship.

The official Court of Inquiry report mentions the two men only once:

“The only survivors who were on board “VANGUARD” when the ship blew up being two men, whose evidence was of no value.”

Very little is known about the lives of Marine J Williams and Stoker 1st Class FW Cox following the disaster.” The Incredible Story Of How Two Men Survived The Sinking Of HMS Vanguard