Scapa Flow covers 324.5 square kilometres of sheltered water enclosed by a ring of Orkney islands - Mainland to the north, Hoy to the west, South Ronaldsay and Burray to the southeast, and Graemsay and Flotta plugging the remaining gaps. The name comes from the Old Norse Skalpafloi, meaning bay of the long isthmus. The Vikings recognised what every admiral since has confirmed: this is one of the finest natural harbours in the world, deep enough for the largest warships, sheltered from Atlantic storms, and positioned to control the northern approaches to the North Sea. From the time of the Norse earls through both world wars, whoever held Scapa Flow controlled the sea lanes between the Atlantic and the North Sea.

After the armistice of November 1918, seventy-four German warships sailed into Scapa Flow to be interned under the terms of the ceasefire. For seven months the ships sat at anchor, their skeleton crews growing restless and resentful, while diplomats in Paris argued over who would get them. Von Reuter, fearing the fleet would be seized and divided among the Allies, gave the order to open the seacocks on 21 June. The timing was deliberate - the British guard squadron had sailed out that morning for exercises. By the time they returned, most of the German ships were sinking or already on the bottom. Nine German sailors were shot and killed by British forces during the scuttle, the last casualties of the First World War. Fifty-two ships sank. Salvage operations eventually raised most of them, but seven remain on the seabed today.

Twenty years later, Scapa Flow's defences were tested again - and found wanting. On the night of 13-14 October 1939, Kapitänleutnant Günther Prien navigated submarine U-47 through Kirk Sound, squeezing between the rusting blockships that were supposed to seal the eastern approaches. He torpedoed the battleship HMS Royal Oak at her anchorage, killing 835 of her crew. It was a catastrophic intelligence and defence failure. The Royal Navy had assumed the blockships were impassable, but Prien proved otherwise. Churchill, then First Lord of the Admiralty, ordered the construction of permanent barriers - massive concrete causeways linking the southern islands, built by Italian prisoners of war. The Churchill Barriers sealed Kirk Sound for good and inadvertently created the road connections that Orkney still uses today.

Seven German warships from the 1919 scuttle remain on the seabed of Scapa Flow: the battleships Markgraf, König, and Kronprinz Wilhelm, and the light cruisers Karlsruhe, Brummer, Dresden, and Cöln. They lie at depths ranging from 12 to 45 metres, in visibility that can reach 20 metres on good days. For recreational divers, this is one of the world's great wreck-diving destinations - the ships are large, intact enough to penetrate, and surrounded by marine life that has colonised the steel hulls over more than a century. The wrecks are also a source of pre-atomic steel, valuable because it was manufactured before nuclear testing contaminated the atmosphere with trace radiation. Salvage companies periodically revisit the wrecks for this low-background steel, used in sensitive scientific instruments.

The naval base closed permanently after the Second World War, and Scapa Flow reverted to what it had been for most of its history: a sheltered anchorage for fishing boats and ferries, ringed by farming islands where cattle outnumber people. The oil terminal on Flotta, which handles crude from North Sea platforms piped ashore via undersea pipeline, is the only industrial presence. From the cliffs above the Flow, you can see the buoys marking some of the German wrecks and, on calm days, the dark shadows of their hulls beneath the surface. Oil still seeps from HMS Royal Oak, forming rainbow slicks that drift across the anchorage. The annual memorial service for her crew is held at the wreck site, a reminder that beneath the quiet waters lie the remains of two world wars and the men who fought them.