From the surface, the water was dark green—the Royal Navy diver peering down from the little boat could see no more than a few feet. Slipping into the sea, he adjusted the air valves on his heavy diving helmet and allowed the weights affixed to his body to drag him down to the soft bottom of the English Channel. Here the visibility, although somewhat obscured, was much better. Looking beside him, he could see they had found his quarry: a freshly sunken submarine, a German U-boat.

Looking up, he could see the reason for its sinking. Swaying balloon-like on their tethers were horned mines scattered about the water. The anchor of the diver’s boat might have hit a mine on its way down, or the diver could have exploded one with his lead-soled boots. But despite all the danger, he was determined to get into that U-boat. He needed to recover its secret documents to help win the Great War.

In 1918, Rear Admiral William Reginald “Blinker” Hall, the Head of Britain’s Naval Intelligence Division (NID) was in need of a new source of intelligence material. NID was in the business of cracking codes—classified communications in which the message’s meaning was replaced by words, phrases, letters, numbers, or other symbols. These missives—military, diplomatic, and naval—told of Imperial Germany’s movements and were invaluable in providing the intelligence necessary to wage a successful war. Almost always, these codes were ciphered, meaning the already-secret messages were encrypted with other letters or symbols transposed over the original to make it even more difficult to crack.

While Blinker Hall’s code breakers could crack these codes given time, the whole business was expedited through the admiral’s covert network of agents who gathered codebooks, cipher keys, and other intelligence material through radio intercepts, captured German vessels, downed zeppelins, spy work, blackmail, and general skullduggery. Armed with these tools, Hall’s code breakers were so capable that they could, in almost real-time, relay the movement of German U-boats, troops, and consular communications to the British government. It was Hall’s code breakers who, in 1917, had deciphered the Zimmerman telegram, in which Germany offered an alliance to Mexico if war broke out between the United States and Germany—one of the contributing reasons for the United States entering the war in 1918.

One of the most important operatives was a janitor in the Imperial German Naval offices who fed cipher keys directly to NID. However, in 1918 he disappeared without a trace, and without a new source of cipher keys, the whole operation slowed and limited the effectiveness of British intelligence. But then the old admiral had an idea.

Hall summoned to his office Lieutenant Commander Guybon Chesney Castell Damant, a 36-year-old gunnery officer from the Isle of Wight who was one of the few commissioned officers in the service who was an expert at deep-sea diving. In 1917, Damant had been tasked by the Admiralty to recover 44 tons of gold bullion (valued in 1917 at $25 million or approximately $1.7 billion today) that had sunk aboard HMS Laurentic to the north of Ireland. Damant’s experiences in the tight confines of the ruined Laurentic made him uniquely qualified for the job Admiral Hall had in mind.

His new mission: command a secret unit of five divers who could wrest Imperial Germany’s codes from the deep. He found it even more exciting than salvaging sunken gold.

In 1917, the Germans had begun a naval offensive against the Allies using U-boats to target Allied shipping. It was at first highly successful, as the innovative weapons surprised merchant ships, sinking them with relative ease, and nearly driving Britain out of the war. By late 1917, anti-submarine measures, including more effective minefields and convoy tactics, started to turn the tide in the Allies' favor. More and more U-boats were being sunk.

Hall realized that the submarines, particularly outbound U-boats from their bases in Belgium, would be carrying aboard the latest cipher keys, code books, and other intelligence material. If Damant could get his Laurentic divers into those wrecks, it could prove to be an intelligence coup.

Starting in April 1918, Damant’s team was tasked by NID to go at a moment’s notice to the location of a freshly sunken U-boat and attempt to get inside. Most of the action occurred in the English Channel, although his group ventured throughout British waters. At first, Damant was summoned to the sites of suspected U-boats only to discover that it was not a useful wreck but either sunken surface ships, rocks, or a U-boat that was too old to be valuable. But on May 20, the team’s luck seemed to change when they located the UB-33 wrecked in 84 feet of water in the English Channel. Then a new set of problems began.

The standard diving dress of the period, which was developed in the late 19th century and refined in the early 20th century, was complex, consisting of a heavy twill suit, corselet, and a cumbersome diving helmet. To prevent a diver from upending, the suit was weighed down with body weights and lead-soled boots. Out of water, the diving gear weighed upwards of 200 awkward pounds. When one of Damant’s men attempted to enter the UB-33 through the narrow conning tower hatch, he could not fit through.

To make matters worse, the wrecked U-boats that Damant’s team explored were generally located in active minefields and were the smaller, coastal variety of submarines rather than the larger oceanic types. Even if a mine went off several miles away, the energy from the blast could rupture a diver’s ears, damage his internal organs, or kill him outright. The U-boats were often loaded with live mines, which added a further layer of danger to the mission.

Damant meditated on how he could get his divers inside the U-boats. At last, he risked using underwater explosives to open up the wrecks. It was efficient, but not safe, and on occasion, the explosives set off secondary explosions that nearly killed the divers. There was a good reason why the Admiralty promised a £500 indemnity to each man’s next of kin.

Once the explosions settled, the divers entered the wrecks. The confines were tight with crushed bulkheads, dislodged batteries that heated the water, and all manner of oily debris that obscured what they could see, even with electric underwater lamps. Since the divers were connected physically to the surface by a lifeline and air hose, there was always a danger of the lines getting caught in the wreckage—or even cut completely.

Then there were ever-present bodies. Marine life scavenged the corpses—conger eels and crabs mostly. There were many cases of apparent suicide and murder—Damant could only speculate on the causes—from last-minute despair that it would be better to take one’s own life than drown, to final revenge against a shipmate or officer. One diver recalled, “I shall never forget the expression of horror upon some of their faces or the mutilated heads of those who had blown out their brains.”

Working over the course of several days, the dive team would dig through a wreck, often scooping out debris by hand from crushed compartments, to recover all sorts of valuable intelligence for NID. Signal books, cipher keys, experimental weapons, and minefield plans were shipped off to London, even as Damant’s section was summoned to the next case. Through the end of the war, November 11, 1918, the divers methodically recovered materials from at least 15 different U-boat wrecks—contributing materially to the Allied cause by supplying intelligence that enabled NID to crack German communications faster, allowing the Allies to shift troops, ships, and materials in a coordinated fashion.

The miracle of it all was that none of these divers were seriously injured or killed. But since their work was highly covert, and the records released decades later, little was ever written about them.  In later years, Damant and his divers were nick-named the “Tin-Openers,” but Damant, who despised publicity, would have infinitely preferred to have been called what they were named during the war, a Special Section of the Royal Navy. It is only now, 100 years later, that the remarkable adventures of these divers are finally surfacing.

Joseph A. Williams is the author of The Sunken Gold: A Story of World War I, Espionage, and the Greatest Treasure Salvage in History.