The destroyers turned back around lunchtime, but Capt Francis Martin-Peake in the Pathfinder continued his patrol.

It was a sedentary sweep by the ten-year-old scouting cruiser, already obsolescent if not obsolete, her top speed of 25kts now reduced to a mere 5kts by a shortage of coal.

In short, Pathfinder was a sitting duck.

Shortly before 3.45pm, Otto Hersing in U21 seized his chance from a range of little over 2,000 yards.

Los! Away. “I fired the first live torpedo in the history of the world,” Hersing rather smugly observed in his memoirs.

Scything through the North Sea at 27kts was a G6 torpedo, 1½ft in diameter, 20ft long and packing 350lb of explosive.

Pathfinder’s bridge team didn’t see Hersing’s periscope, but they did spot the tell-tale streak of a torpedo’s wake.

At 2,000 yards, it gave the cruiser just over two minutes to react. React she did. One engine full astern, the other full ahead, the rudder hard over.

To no avail. Hersing’s torpedo struck Pathfinder beneath her bridge, piercing the two-inch-thick armour.

The impact alone and the hole it left may have done for the cruiser, but a second cataclysmic explosion tore the ship apart – in all likelihood the fires caused by the torpedo ignited her magazine, throwing columns of black and white smoke into the air.

Pathfinder went down in four minutes, her death quick, but violent. Her stern was thrust out of the North Sea at an implausible 60 degrees. Her bow broke away under the strain. The fore mast and fore funnel collapsed. A stern 4in gun came off of its mounting and spiralled into the water, taken its crew with it.

For those below there was no hope. For those on the upper deck little, for there was no time to put the boats away – and almost no flotsam to cling to in waters no warmer than 60˚F.

As a result, the death toll was terrible. No more than 18 souls survived the loss of the Pathfinder. Two hundred and sixty-one officers and men – the latter almost all Chatham ratings.

Destroyers closed on the scene, while the lifeboat from St Abbs on the south shore of the firth was launched. Its crew returned with a story of horror, as recounted to the budding novelist Aldous Huxley, recuperating in Scotland from illness.

“There was not a piece of wood, they said, big enough to float a man – and over acres the sea was covered with fragments – human and otherwise. They brought back a sailor’s cap with half a man’s head inside it.”

There were, as far as we’re aware, no photographs of HMS Pathfinder’s sudden and horrific demise. But the tragedy was recorded on canvas by William Lionel Wyllie, the pre-eminent Naval artist of his age. (Incidentally, one Stoker 1st Class William Wyllie went down with the cruiser.)

Publicly, the Admiralty would claim – with some degree of success – that Pathfinder had fallen victim to a German mine (indeed the six RN vessels lost in the first month of war had done just that).

And privately, many Naval officers believed the official line – although Francis Martin-Peake corrected them; Pathfinder’s commanding officer was one of the few survivors.

The Commander-in-Chief of the Grand Fleet, Admiral Jellicoe, took Martin-Peake at his word – and took the U-boat threat seriously, almost to the point of paranoia.

Yet too many of Jellicoe’s contemporaries did not. Hubris inevitably leads to nemesis – but it was the flower of Britain’s youth who would pay the ultimate price, not the men laden with gold braid.

As for Otto Hersing, the 29-year-old Alsatian would prove to be one of the Kaiser’s most adept U-boat commanders.

He would sink three more warships – earning him the nickname Zerstörer der Schlachtschiffe (Destroyer of Battleships) – and three dozen merchantmen before the war’s end.

News of his historic success against the Pathfinder would not be announced until late September.

By then, the German people had another U-boat hero to hail – and Britain a far worse naval tragedy to mourn.