The clock is ticking in the all-hands-on-deck search for the tourist submersible that went missing during a deep-sea dive to the Titanic shipwreck on Sunday.

The vessel has five people on board and, according to the U.S. Coast Guard, a dwindling oxygen supply of 40 hours.

That gives responders just two days to locate the Titan — which is believed to be hundreds of miles from the nearest coast and potentially thousands of feet below sea level — plus bring it back to the surface to rescue those inside.

It's a complex mission, with retired U.S. Navy submarine Capt. David Marquet putting the odds of passengers' survival at "about 1 percent."

And it's certainly not the first of its kind: There have been several prominent rescue missions for both submarines and submersibles (which are not fully autonomous) over the course of the last century.

The deepest underwater rescue ever accomplished, officially, was that of the commercial submarine Pisces III, off the coast of Ireland in 1973.

In that dramatic incident, two crewmen — both named Roger — spent three days trapped in a vessel measuring 6 feet in diameter, subsisting off a single sandwich and condensation licked from the walls, until they were rescued with just 12 minutes of oxygen to spare.

One of them, Roger Mallinson, told NBC News on Tuesday that the search for the Titan has evoked tough memories of his own experience.

"You just rely," he said, "on the thing being well-made."

The submersible after a routine dive

It was August 1973, and two British sailors were heading out on a routine dive to lay transatlantic telephone cable on the seabed about 150 miles southwest of Cork.

Senior pilot Mallinson, an engineer, was 35 at the time. Former Royal Navy submariner Roger Chapman, who died in 2020, was 28. They were clocking eight-hour shifts, crammed into a small vessel with very poor visibility, according to the BBC.

On the morning of August 29, as the two were getting ready to be towed back to their mother ship, a hatch was accidentally pulled open. Water flooded a self-contained part of the submersible, adding extra weight and plunging the vessel about 1,575 feet below sea level.

"There was lots of banging of ropes and shackles — as normal during the last phase of the operation — when suddenly we were hurtled backward and sank rapidly," Chapman told the BBC in 2013. "We were dangling upside down, then heaved up like a big dipper."

The two hastily prepared to crash, dropping a lead weight to lighten their load, curling up in safety positions and stuffing cloth in their mouths so as not to bite their tongues off. They hit the ground in about 30 seconds, at 40 miles per hour.

They weren't injured, but they were stuck.

They had to conserve oxygen and food

Author Stephen McGinty, who recounted the rescue in his book The Dive (which is reportedly being made into an action movie), explained the severity of the situation in a 2021 Newsweek interview.

"Try to imagine you are in a phone box with a friend, the phone box is at the bottom of the Empire State Building, then everything around you floods to ten stories above the top of the Empire State Building," he said. "Then turn out all the lights and start bleeding oxygen, then you realize that a rescue — if it can even be attempted — is roughly two days away."

Mallinson and Chapman didn't have a water supply, just one can of lemonade and a cheese sandwich, which they wanted to save for later.

By a stroke of luck, Mallinson had replaced the oxygen tank just before the dive — but they only had 66 hours left.

The two decided to conserve oxygen by doing as little as possible. Once they telephoned for help and made sure the nearly upside-down vessel was in order, they didn't talk or move.

They lay in the pitch-black submersible as high up as possible, where the air quality was better, thinking about their families.

"We hardly spoke, just grabbing each other's hand and giving it a squeeze to show we were alright," Mallinson told the BBC. "It was very cold — we were wet through."

The rescue operations suffered a series of setbacks

Meanwhile, an international rescue operation was underway, involving dive teams from the United Kingdom, Canada and the U.S.

"The plan was relatively simple: a sister sub would go down with a two-man crew and attach a specially designed grapple hook to the sub then lift it to the surface," McGinty explained. "But they do say: how do you make God laugh? Tell him your plans."

McGinty said the floating buoy that ran on a rope from the surface had been disconnected from the submersible several minutes before it sank, so crews knew "where the haystack was, just not the needle." They were able to detect the vessel using sonar by making Chapman sing — "in the hope of picking up the high notes."

Then they had to actually reach it. Multiple attempts to raise the submersible failed over the next two days, leaving the responders with two broken vessels and the passengers without much hope.

"The first sub to go down lost its lift line; the second sub down couldn't find them," McGinty said. "On a third trip they finally found Pisces III, but when they attempted to fix the lift line it locked on then fell out."

On Sept. 1, a team was finally able to make repairs to one of the other submersibles and send it back down, where it managed to attach a tow rope to the vessel.

Chapman told the BBC that it was only once the pilots knew the line was safely attached that they had the sandwich and lemonade. Mallinson later wrote that "it tasted like champagne to us."

The lift itself proved difficult and had to be stopped and restarted twice, with lots of swinging around. The crew described the ride up as disorienting, with Chapman saying rescuers "thought we'd died when they looked at us, it had been so violent."

Once they made it to the surface, it took them about half an hour to open the hatch and get fresh air. And there hadn't been a moment to waste.

"We had 72 hours of life support when we started the dive so we managed to eke out a further 12.5 hours," Chapman said. "When we looked in the cylinder, we had 12 minutes of oxygen left."

The incident left a lasting impact on both survivors

The doctor who examined the pair commented "incredible," McGinty said. They were dehydrated, and Mallinson had mild hypothermia, but they were otherwise in good shape.

The incident left a lasting impact on both Mallinson and Chapman in other ways, including forming a lifelong bond.

"Each year on the anniversary Roger Mallinson would call Roger Chapman at the exact moment they reached the surface," McGinty said.

Chapman went on to set up a company specializing in submersible rescues and was able to help with several incidents, according to his obituary. The "grandfather of submarine rescue" said even years later that he occasionally felt uncomfortable in elevators.

Mallinson, who became renowned for his work on steam engines, was awarded an MBE at the beginning of 2023.

In a September 1973 Daily Mail column, Mallinson wrote that he owed his life to Chapman.

"The ex-Navy lieutenant, who was my second pilot and observer aboard the stricken Pisces III, pulled me through the blackest hours of that incredible rescue," he wrote. "Without him, I would not be here to tell this story."

Few other sub rescues have been as successful

The Pisces III incident took its place in the history books as the deepest underwater rescue ever achieved, according to Guinness World Records. Many others have been attempted, with varying degrees of success.

Take for example the USS Squalus, a submarine that sank 240 feet off the coast of New Hampshire during a test dive in 1939, killing 26 people immediately.

The remaining 32 crew members and one civilian used smoke bombs and, later, morse code to signal for help. A Navy submarine found them that same morning, and rescuers were able to bring the survivors to the surface in four separate trips over the next day or so. It took another three months to recover the vessel, by attaching pontoons to both sides and inflating them full of air.

Russia saw one of the world's worst naval disasters several decades later, in 2000, when the nuclear submarine Kursk sank during a training exercise in the Arctic Circle. All 118 crew members ultimately died, though some two dozen had survived the initial sinking.

The Russian government — led by newly minted President Vladimir Putin — was slow to launch search and rescue efforts, even rejecting offers of help from Western countries. By the time a team of British and Norwegian divers found the vessel nine days later, there were no survivors.

Five years later, when the Russian AS-28 sank in the Pacific Ocean after becoming entangled in fishing nets, the government took a different tack and called for international help. British and American rescue crews were able to free the vessel and save all seven people on board.

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