As we sat on board a retrofitted fishing boat drifting where the North Sea meets the Atlantic, each member of our group ran through a pre-dive checklist. Yes, my dry suit and vest inflated. The 12kg of weights on my hips to keep me submerged were unmistakably present. I sucked on my regulators. My air worked fine.

But even as we went through the kind of safety procedure that can stave off – or at least mitigate – underwater disaster, my mind was already deep below the surface.

Thirty metres below, to be exact, to where the wreck of a 115m-long, 5,531-tonne German light cruiser from World War I, its 150mm guns long silenced by seaweed, lay visible only to divers and fish. Anywhere else – and certainly in warmer, clearer waters – a wreck with the eerie lure of the SMS Dresden would be near-swamped with divers. But here, in Scotland’s Scapa Flow, it’s just one of many military secrets lurking silently below the surface.

To get there, though, we would have to descend through water that topped out at 12C. That might not sound too painful, but you cool 25 times faster in water than in air and dry suits were necessary to stave off hypothermia.

This might make diving Scapa Flow, a 325sqkm natural harbour penned in by the islands of Orkney, sound inhospitable. But dive enthusiasts around the world know that, even in the cold, Scapa Flow is one of Europe’s greatest dive hotspots.

On the surface, Orkney – a smattering of 70 islands that start 10 kilometres north of Scotland’s northern tip – is one of the most peaceful places imaginable. Farms roll across green hills so vibrant they look stroked on with pastels. The islands host more cows (30,000) than people (21,000); the largest town, Kirkwall, claims only 9,000 residents. Aside from the sky, which can shift in seconds from bright-blue to fierce cloud, and the sea, which shines turquoise one moment, choppy and slate-grey the next, the islands offer little drama.

Delve deeper, though, and that tranquillity collapses. On the west coast of Orkney’s main island, a seaside ramble that edges past a golf course, cemetery and sheep farm also passes a hollowed-out, concrete pillbox – one of the remnants of Ness Battery, used in both world wars and until 1955 armed with two 152mm calibre guns. On the tiny, now-uninhabited island of Lamb Holm, the elaborately decorated Italian Chapel was built entirely by Italian prisoners in World War II; the tabernacle’s wood came from a shipwreck, the rood-screen from scrap metal, the head of Christ made from local red clay. Poke among the farms, meanwhile, and you’ll find radar stations and airfields dotted among the sheep.

In both World War I and II, Scapa Flow was the main base for Britain’s Royal Navy. The location meant the British could help keep the German fleet penned in to the Baltic Sea – and Scapa Flow, the world’s largest natural harbour after Sydney’s, served as protection for British ships. From 1914 to 1918, and again from 1939 to 1945, the waters around Orkney swirled with submarines and churned with ships. In June 1916, just west of Orkney, a German mine blew up the HMS Hampshire, killing all but 12 of the 655 men aboard – including Lord Kitchener, Britain’s Minister of War. And it was from Scapa Flow that the British fleet sailed to hunt the Bismarck, sinking the gem of the German navy in May 1941.

Now, as we descended through the cool water, the wreck of the SMS Dresden appeared like a ghost of the deep through the water’s 10m visibility.

The SMS Dresden is one of seven German wrecks sunk at the end of World War I that are still diveable in Scapa Flow. The reason they sank here is part strategy, part folly. When the armistice for World War I was signed in November 1918, part of the agreement was that Germany give up its High Seas Fleet, kept captive in Scapa Flow. The armistice was due to expire on 21 June 1919. But one person who didn’t know about its last-minute, two-day extension – as all of his information came from four-day-old newspapers – was the German fleet’s Admiral von Reuter.

The hull was covered in barnacles, rust and algae. Fish hovered near the cylindrical Armoured Control Tower, from where crew members would have navigated during battle, protected by a 101mm steel plate. We swam over the boiler rooms, the hatches broken open decades ago. In a place that once must have heaved with the sound of shouting men and furnaces, all I could hear was the whoosh of each breath I drew from the tank.

That day, fearing the fleet would fall into Allied hands, he ordered all 74 ships scuttled. Flood valves and seacocks were opened, portholes loosened, water pipes smashed. Although the shocked Brits were able to beach many of the ships, 52 of them sank to the sea floor.

Most were salvaged, seen as sources of scrap metal. The seven that remain on the seabed today – three light cruisers, three battleships and one fast mine-layer – are protected. Divers can explore them, but they and anyone else are forbidden from taking anything at all. Even with that protection, though, the ships are deteriorating.

And they are just a fraction of the history that litters Scapa Flow. More than 150 other wrecks are scattered on the seabed, from 19th-century schooners to German U-boats to what’s thought to be a Spitfire.

Between the dives, which I did as part of the advanced diving course at Scapa Scuba Dive Centre, we stopped at the Visitor Centre and Museum on the island of Hoy, the former location of the naval base. Once full of fuel for the British fleet, the former pump house is today full of exhibits and objects from both world wars, including many salvaged from the wrecks.

I found myself perusing a binder of documents belonging to one John Milligan, aged 19. The letters to his mother about life in the Royal Navy came to an abrupt halt with a telegram of his death on the HMS Royal Oak, torpedoed in Scapa Flow by the German submarine U-47 in October 1939. Of those aboard, 834 were killed, many of them as young as John Milligan. Like other war graves at Orkney, the Royal Oak cannot be dived.

That afternoon we dived the 150.8m-long, 5,440-tonne SMS Karlsruhe, another of the wrecked German ships. At 25 metres underwater, we watched anemones swirl in the slight current at the ship’s bow. Sea urchins and starfish clung to the remains of the bridge; a crab scuttled across splintered teak planks of the deck. We ducked beneath the enormous shafts of rust-covered anchor capstans. Where the boiler rooms had been, there were fossilised chunks of coal.

Turning, we followed the hull’s port side back to the buoy line that would lead us back up to our start. By the time we surfaced, we’d been below for only 31 minutes, but it was enough time to explore a German light cruiser from bow to stern – and enough time to wind the clock back by nearly a century and explore a once-powerful ship that, today, only those venturing to the deep could see.

This story is a part of BBC Britain – a new series focused on exploring this extraordinary island, one story at a time. Follow us on Facebook, Twitter and Instagram.

