At 3:25 pm on 30 December 1915 the armoured cruiser HMS Natal, then moored in the Cromarty Firth signalled that she was on fire. Other ships were ordered to give assistance, but at 3:30 pm she turned over and had sunk by 3:45 pm: timings are from the website hmsnatal.co.uk.
There is some doubt about the number of dead and survivors. Wikipedia says that some of her crew were not on board at the time of the sinking as they had been given shore leave to either play in or watch a football match. It gives a range of 390-421 for the number of dead. The Commonwealth War Graves Commission website says that there were seven wives of officers, three children, a civilian and some nurses from the Hospital Ship Drina on board, attending a Christmas film show. It says that over 420 people died, including 414 naval personnel.
hmsnatal.co.uk states that 421 died. Its list of the dead includes 423 names, but at least one person is double counted, with Commander John Hutchings’s wife Mabel being included under both her married name and her maiden name of Cuningham. The list also includes Mrs Violet Back and Mrs Bennett, the wives of Captain Eric Back and Engineer Lieutenant Frank Bennett respectively. All three husbands also died. Mr Dodd, the Factor of the nearby Novar Estate, his wife and their three children were also amongst the dead, as were Nursing Sisters Caroline Edwards, Eliza Millicent and Olive Rowlett of Queen Alexandria’s Royal Naval Nursing Service.
The total dead would therefore appear to be 422: eight civilians, including four women and three children, and 414 naval personnel, including three nurses.
It was originally thought that Natal had been sunk by a U-boat, but it was later realised that her loss resulted from an accidental explosion of her ammunition. She was the second British armoured cruiser to be lost accidentally in nine weeks: HMS Argyll ran aground in heavy seas on the Bell Rock, near Dundee, on 28 October. All her crew were saved, but she was totally wrecked.
On 8 December a collision between the new Queen Elizabeth class super dreadnoughts HMS Barham and Warspite left both requiring dockyard repairs, reducing the British margin of superiority in the North Sea. The British thought that they needed a big margin over the Germans because they assumed that the German High Seas Fleet would only come out when at full strength, whereas their Grand Fleet would be reduced by repairs and refits. In fact, the German battlecruiser SMS Von der Tann missed the Battle of Dogger Bank because she was in dry dock and the Germans were not at full strength at the Battle of Jutland in 1916.
Part of Natal’s wreck remained visible and was saluted by RN ships entering or leaving the Cromarty Firth. Plans to salvage in the 1920s and 1930s did not come to fruition. In the 1970s the wreck was reduced in size to prevent it being a danger to shipping: see the website of the Royal Commission on Ancient and Historic Monuments of Scotland. The development of North Sea oil would by then have increased civilian shipping traffic in the area.
Natal was not the first British warship to be lost to an accidental explosion whilst moored, suggesting that RN ammunition handling procedures were lax. The pre-dreadnought battleship HMS Bulwark blew up on 26 November 1914 and HMS Princess Irene, a passenger line converted into an auxiliary minelayer, exploded on 27 May 1915. The dreadnought HMS Vanguard and the monitor HMS Glatton both blew up later in the war.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. iii, p. 261