The mass escape of 76 Allied airmen from a Nazi POW camp in March 1944 remains one of history’s most famous prison breaks. Although the German Luftwaffe designed the Stalag Luft III camp to be escape-proof, the audacious, real-life prison break immortalized in the 1963 movie The Great Escape proved otherwise.

When the Nazis built the maximum-security camp 100 miles southeast of Berlin to house Allied aviators captured in World War II—many of whom had made previous escapes—they took elaborate measures to prevent tunneling, such as raising prisoners’ huts off the ground and burying microphones nine feet underground along the camp’s perimeter fencing. In addition, the camp was built atop yellow sand that would be tough to tunnel through and difficult to conceal by anyone who tried.

The Nazis, however, didn’t account for the daring and ingenuity of the British, American Canadian and other Allied flyboys who toiled for nearly a year to construct a tunnel that would allow them to flee from captivity. For the aviators, the penalty for being caught trying to escape—generally 10 days in solitary confinement under the rules of the Geneva Convention—was worth the risk.

The secret operation was led and organized by Roger Bushell, a Royal Air Force pilot who had been shot down over France while assisting with the evacuation of Dunkirk. In the spring of 1943, Bushell and over 600 prisoners of war began building three tunnels with the code names of Tom, Dick and Harry. The plan called for each tunnel to stretch for more than 300 feet to the protective cover of the forest outside the camp’s perimeter fence.

Inside Hut 104, the prisoners building the Harry tunnel toiled for days chipping away at the building’s support columns to avoid being seen working underneath the barracks. From a trap door concealed below a heating stove always kept lit to discourage the Nazi guards from getting too close, they burrowed down more than 30 feet to be out of the range of the microphones. Working in claustrophobic conditions, diggers stripped to their long johns or took off all their clothes so that the bright golden sand wouldn’t stain them and raise the suspicions of the German guards. The captives excavated at least 100 tons of sand, which they stuffed into concealed socks and discreetly sprinkled and raked into the soil of the small gardens tended by the prisoners.

Scavenging and stealing materials for the operation, the prisoners stripped some 4,000 wooden bed boards to build ladders and shore up the sandy walls of the two-foot-wide tunnels to prevent their collapse. They stuffed 1,700 blankets against the walls to muffle sounds. They converted more than 1,400 powdered milk tin cans provided by the Red Cross into digging tools and lamps in which wicks fashioned from pajama cords were burned in mutton fat skimmed off the greasy soup they were served.

As the tunnel lengthened and oxygen levels fell, the prisoners used a stolen wire to hook up to the camp’s electrical supply and power a string of light bulbs. They even fashioned a crude bellows-type air pump system built in part with hockey sticks, knapsacks and ping pong paddles. And they constructed an underground trolley system pulled by ropes to transport the sand with switchover junctions named after two London landmarks—Piccadilly Circus and Leicester Square.

To prevent the Nazis from learning of the operation, the airmen employed an elaborate lookout system and used subtle signs such as turning the page of a book or fiddling with a shoelace to raise notice of an approaching guard. By bribing guards with Red Cross goods unavailable in Germany—such as chocolate, coffee, soap and sugar—prisoners obtained cameras and travel documents that a team of artists used to forge identity cards, passports and travel passes. They replicated travel stamps by carving patterns in boot heels and using shoe polish as ink. The plan was to break out some 200 POWs, chosen by who had the best language and escape skills to succeed, who worked most in the preparation and, then, by lottery.

The Nazis eventually discovered the tunnel Tom and summoned photographers to chronicle their find before its demolition. While the Nazis celebrated their discovery, however, they were unaware that work on the two other underground passages continued. The prisoners eventually turned Dick into a storage space and focused all construction on Harry, which was completed at the end of winter in 1944.

Around 10:30 pm. on the frigid, moonless night of March 24, 1944, British bomber pilot Johnny Bull slowly traversed the tunnel more than 30 feet below the oblivious Nazi guards and peeked his head out of the snowy ground beyond the camp’s fence. As he breathed in the glacial air and filled his lungs with freedom, the sweat-soaked prisoner discovered that the tunnel had stopped feet short of the protective cover of the forest. The blunder slowed the escape process—those emerging from the tunnel had to wait for a “coast clear” rope-tug signal from an escapee already in the forest—and dashed plans to break out the full 200 men.

The process was tedious as the prisoners, dressed in civilian clothes and carrying forged documents, lay down on the rope-operated wooden trolley and were pulled one-by-one through the tunnel to their escapes. Fewer than a dozen men made it through every hour, and a partial tunnel collapse and a one-hour blackout during a midnight air raid further slowed the operation.

Around 5 a.m., a German soldier on patrol nearly fell into the exit shaft and discovered the tunnel. The prisoners inside scrambled back to the hut and burned their forged documents. The Nazis discovered that 76 prisoners had broken out of their supposed escape-proof camp.

The audacity and resourcefulness demonstrated by the Allied pilots was the stuff movies are made of, and the breakout was immortalized in the 1963 blockbuster The Great Escape, which starred Steve McQueen, James Garner, Richard Attenborough, Charles Bronson and James Coburn. There was no Hollywood ending, however, for most of the 76 men who broke out of Stalag Luft III.

The Nazis mobilized a massive manhunt. They erected roadblocks, increased border patrols and searched hotels and farms. Within two weeks, the Germans had recaptured 73 of the escapees. Only three men successfully fled to safety—two Norwegians who stowed away on a freighter to Sweden and a Dutchman who made it to Gibraltar by rail and foot.

A furious Adolf Hitler personally ordered the execution of 50 of the escapees as a warning to other prisoners. In violation of the Geneva Convention, Gestapo agents drove the airmen—including Bushell and Bull—to remote locations and murdered them. Following the war, British investigators brought the Gestapo killers to justice. In 1947, a military tribunal found 18 Nazis guilty of war crimes for shooting the recaptured prisoners of war, and 13 of them were executed.