On 21 June, 1919, at 11am, 74 captured German warships were scuttled on command of Admiral Ludwig von Reuter and sent to the bottom of Scapa Flow in an act that thwarted the Allies' intention to lay claim to the defeated German navy.
The German fleet had been held captive at the British naval base in Orkney's Scapa Flow since the First World War ended in November 1918, and the crews had not been allowed off the ships. Morale was at its lowest ebb and the situation was a powder ke
g. It came to a head when, following a secret code issued by the admiral, the crews opened the stopcocks of their vessels and sank them.
The majority were salvaged for scrap between the 1920s and 1970s – until their potential in terms of historical value was eventually realised. Now, as the 90th anniversary of the scuttling approaches, only seven remain in their watery resting places, helping to make Scapa Flow one of the best dive sites in the world.
Battleships Konig, Kronprinz Wilhelm and Markgraf and light cruisers Brummer, Dresden, Koln and Karlsruhe attract thousands of sub-aqua tourists to Orkney each year.
For marine scientist and diving archaeologist Bobby Forbes, their value in terms of maritime heritage is immense.
"They represent a significant episode in British maritime history and the fact that they're there brings the past into the present – but, after 90 years on the seabed, the inevitable decay is taking its toll and they're not going to be there for ever."
The importance of the Scapa Flow wrecks has led to the formation of ScapaMAP (Scapa Flow Marine Archaeological Project) – involving government agencies, industry and the academic community. New technology has ensured that highly accurate images of the wrecks and the surrounding area are now digitally stored for posterity, and Forbes is currently organising training for recreational divers under the auspices of the Nautical Archaeological Society.
"There aren't many diving archaeologists about," he says, "and most finds are made by recreational divers, so it's vital to show them how to carry out the correct survey techniques."
This point was made a few years ago in Scapa Flow, when a group of recreational divers, already overawed by the sight of the Karlsruhe looming out of the murky depths, became aware of what looked like scraps of material drifting from a lump of sediment.
The scraps were part of a waterlogged collection of postcards intended for German wives and sweethearts, to reassure them their husbands and lovers were safe and well.
Back on shore, the divers reported what they'd found, and Historic Scotland contacted Bobby Forbes to try to recover as much of the material as he could from the Karlsruhe. He got together a team of six and prepared to do an underwater "dig".
He found all his skills tested to the limit when it came to removing the postcards from the wreck.
"The reason they'd survived so long was because they'd been buried in sediment and there had been no water movement around them until the ship's bulkhead had started to corrode," he says. "They'd been in tin boxes and the deteriorating metal had formed a concretion with the bulkhead metal, so getting them out was a bit of a nightmare, to put it mildly."
Mandy Clydesdale, a conservation consultant with AOC Archaeology, a Midlothian company that specialises in restoring valuable artefacts, including books and papers, says: "The fact that the postcards have survived at all is nothing short of amazing.
"The more I worked with these postcards, the more I appreciated how skilful these divers had been in getting them from the wreck to the lab. The postcards were so fragile that trying to handle them was like trying to lift slices of yoghurt."
After months of painstaking work, the restored postcards were ready to be displayed to the public, and they can be found in the archive department at the Kirkwall branch of the Orkney Library.
The scene on one is informal and homely – a little girl in pigtails bends purposefully over something half written, while her brother lies back against a cushion, smiling over the contents of a letter in his hand. It's a world away from the ghastly realities of the First World War, and the kind of image that would remind sailors of the homes they were fighting for.
A second card shows a wounded officer in dress uniform with head held high and hero worshipping maiden in attendance.
Other scenes show flotillas of ships butting their way through choppy seas.
Previous generations had no scruples about appropriating "scrap" they considered to be fair game on the seabed, but somehow the post-cards remained untouched in their metal containers.
And 90 years on, time has stood still for a little boy and girl writing letters to Papa.
• The postcards are in the Orkney Library archives,
www.orkneylibrary.org.uk; the sonar images of the wrecks and their history can be seen at
www.scapamap.org