33 men at 73 metres and nobody had ever come up from there

08:40, 23 May 1939, off the Isles of Shoals, a cluster of islets level with Portsmouth on the coast of New Hampshire. The submarine Squalus, of the United States Navy, delivered less than three months earlier, begins her nineteenth test dive. There are fifty-nine people aboard: fifty-six sailors and three civilians from the Portsmouth navy yard who have come along to check the machinery. In command is Lieutenant Oliver Naquin, thirty-five. The manoeuvre they are about to rehearse is the one any submarine will have to perform in war: going from surface to full dive in under a minute.

Five minutes later. The control board on the bridge shows green: every opening shut. They are not. The main induction valve — the duct almost a metre across through which the diesels suck air when the boat runs on the surface — has not closed. The sea is pouring through it into the after engine room at tonnes per minute. Inside, the noise is a punch of compressed air that blocks the ears.

Naquin orders the watertight doors shut. The doors close against the shipmates on the other side. The bulkheads seal off the flooded stern from the dry bow. Twenty-six men are left behind those doors.

Ten minutes later. The Squalus settles on the bottom with her stern buried in the silt. The depth is about two hundred and forty feet: seventy-three metres. Up to that day, no crew of any country had come out alive from a submarine sunk at that depth. The emergency breathing bag the Navy itself issued — the Momsen lung — had been tested to about two hundred and seven feet, sixty-three metres, and always with trained, rested men; the men below had been hours in the cold and the foul air. For seventy-three metres there was no written procedure and no precedent at all.

First hour on the bottom. The thirty-three survivors are spread between the forward torpedo room and the forward compartments. Naquin has signal rockets fired, which break the surface and go out, and releases the marker buoy: a float joined to the hull by a cable carrying a telephone line. Then he orders that nobody move more than necessary, that the lights go out, and that soda lime be spread on the deck. Soda lime absorbs the carbon dioxide they breathe out; the less they move, the less they produce. It is cold, and the cold will deepen all night.

Midday on the 23rd, aboard the Sculpin. The sister submarine, sent out from Portsmouth to look for them, sights one of the rockets and then the buoy. A sailor climbs down, opens the lid and lifts the telephone. Somebody answers. The conversation lasts as long as it lasts: the Sculpin's officer is told the stern is flooded, Naquin comes on the line, says "Hello, Wilkin", and the cable parts in the swell. In that half minute the surface has learnt that there are men alive, and where. What pins the spot down so it cannot be lost again is the tug Penacook, dragging a grapnel — the four-pronged anchor used to drag a seabed — until it snags the hull.

Afternoon of the 23rd, at New London. The Falcon sails from the Connecticut yard: a salvage ship with a crane, a decompression chamber and, on deck, a device never used in a real emergency — the McCann rescue chamber. It is a ten-tonne steel bell, pear-shaped, divided into two compartments; it is lowered onto the submarine's hatch along a cable that a diver has to shackle on by hand down below, and it seals against the hatch ring by blowing air to force the water out. It carries the name of the officer who finished it, Allan McCann, and grows out of another man's idea: Charles Momsen, known as Swede, who has spent ten years insisting that a sunken submariner can be gone after. Momsen boards the Falcon to run the diving. The device has worked in trials, with volunteers and good weather. In an emergency, never.

Night of the 23rd to the 24th. Inside the Squalus the temperature drops to near freezing. The men climb into the bunks two at a time to keep warm, with blankets and wet working clothes. They speak little, because speaking uses air. Some pray, some sleep. Naquin walks the compartments at intervals checking who is shivering and who is not, because the man who has stopped shivering is the one in trouble.

Around three in the morning. On the surface they hear banging coming up through the water and take a moment to understand that it is Morse. It is a chief petty officer aboard the Squalus striking the hull with a wrench. The message he sends, written into the records of that night, is four words: "Condition satisfactory but cold." For the hours that follow, that wrench and a hammer are the only telephone left: the men below and the men above talk to each other in blows against the steel.

Small hours of 24 May. The Falcon arrives overhead and drops four anchors to hold herself still above the submarine. The divers' work begins: going down to the submarine's deck in helmet and canvas suit, breathing air pumped from the surface through a hose.

Mid-morning on the 24th. Diver Martin Sibitzky reaches the deck of the Squalus, gropes for the forward escape hatch and shackles the main cable to its ring. That cable is the whole invention: the chamber runs down it like a bead on a thread, and arrives at the right hatch instead of the deck plating beside it.

Midday on the 24th. The chamber is swung off the Falcon with two men inside. It goes down slowly, seats itself over the hatch, blows air to force the water out of its lower compartment and makes a seal. Only then can the submarine's hatch be opened without the sea coming in. This is how the thirty-three get out: sitting inside a steel bell sealed over the hatch, in loads of seven to nine, while the Falcon hauls on the cable.

Afternoon of the 24th. The first trip brings seven men to the surface. They have been more than a day on the bottom. The second trip lifts nine. The third, nine more: twenty-five in three trips that go smoothly. Eight are left below with Naquin, who has decided to be the last man out.

Night of the 24th. The fourth trip is the one that nearly ruins everything. The chamber loads the last eight and starts up; at a hundred and sixty feet — about fifty metres — the downhaul cable frays and jams in the mechanism. The chamber has to go back down to the submarine and wait there, with eight men and two operators inside, while the surface decides what to do. There is no motor that will lift it. Momsen orders it done by hand: the Falcon's sailors form a line and haul the wire in hand over hand, in relays.

After midnight, now 25 May. The chamber breaks the surface with the last eight. Thirty-three men, four trips, not one casualty in the rescue. It is the first successful salvation of a crew from a sunken submarine in the history of seafaring, and it remains the pattern on which today's rescue vehicles are designed.

Afterwards. The twenty-six in the stern could not be recovered until the salvage. The Navy awarded the Medal of Honor to four divers — William Badders, Orson Crandall, James McDonald and John Mihalowski — all four for the work of those hours. The Squalus was lifted from the bottom on 13 September 1939, towed to Portsmouth, repaired and returned to service under another name, Sailfish, so that nobody would have to sail in a cursed ship; during the war she sank seven Japanese vessels. The sailors always called her Squailfish.