Forty years ago two British sailors plunged almost 1,600ft into an abyss, 150 miles off Ireland, in a deep-sea submersible. Trapped in a 6ft-diameter steel ball for three days, the men had only 12 minutes of oxygen left when they were finally rescued.

The story of Pisces III, which made headlines at the time, is now largely forgotten.

But on Wednesday 29 August 1973 former Royal Navy submariner Roger Chapman, then 28, and engineer Roger Mallinson, then 35, plunged to the bottom of the Atlantic ocean in an accident, sparking a 76-hour international rescue operation.

Here's how the incident and rescue effort unfolded:

01:15 - Dive begins

Image caption The brown square is where Pisces went down

Pilot Roger Chapman and senior pilot Roger Mallinson commenced a routine dive in Pisces III. The Canadian commercial submersible - working on a charter for the Post Office - was laying transatlantic telephone cable on the seabed 150 miles south west of Cork.

"It took about 40 minutes to sink down to not far off 1,600ft (500m) and a bit faster to get back up," says Chapman. "We'd do eight-hour shifts, going along the surface of the seabed at half a mile an hour, setting up pumps and jets which liquefied the mud, laying cable and making sure it was all covered. It was very slow, murky work."

Mallinson says the poor visibility made the job tiring. "It was like driving down the motorway in thick fog and trying to follow a white line - you had to concentrate beyond belief. One pilot would have the controls for the sub in one hand and the manipulator - a mechanical hand, which would lift, twist, extend and move sideways - in the other, then we'd swap," he says. "It was also uncomfortable. We had to kneel, with our heads by our knees."

For Mallinson, the shift followed a 26 hour-stint without sleep. "A previous dive had damaged the manipulator so I worked through the day repairing it. I knew Pisces III inside out as I'd rebuilt it when it came over from Canada as a wreck," he says.

By a stroke of luck, the engineer also decided to change the oxygen tank. "It was quite ample to run the dive, but for some reason I decided to change it to a full one, which was no mean physical feat as it was very heavy. I could have got into trouble for changing a half used bottle, but as it happens, if I hadn't, we wouldn't have lived."

As well as laying cable, the pilots had to look after life support. Every 40 minutes they turned on a lithium hydroxide fan to soak up the carbon dioxide they breathed out and then fed in a small quantity of oxygen.

They also kept a video commentary during every dive for records.

09:18 - The accident

Pisces III was on the surface being recovered.

"We were waiting for the towline to be attached to lift us and take us back to the mother ship. There was lots of banging of ropes and shackles - as normal during the last phase of the operation - when suddenly we were hurtled backwards and sank rapidly. We were dangling upside down, then heaved up like a big dipper," says Chapman.

The aft sphere - a smaller watertight sphere where the machinery was - had flooded when the hatch was pulled off. Suddenly the sub was over a tonne heavier. "As we sank my biggest worry was whether we were anywhere near the continental shelf because if we hit it we'd be crushed."

Mallinson says the sub was jolting, with everything breaking loose, as they went down. "It was very frightening - like a stuka dive bomber with screaming motors and the pressure gauges spinning around."

The pair shut the electrical systems and switched everything off so it was pitch black, dropping a 400lb (181kg) lead weight to make it lighter as they descended.

"It was about 30 seconds until we hit. We turned the depth gauge off at 500ft (152m) as it could have burst and got cushions and curled ourselves up to try and prevent injuries. We managed to find some white cloth to put in our mouths so we didn't bite our tongues off too," says Mallinson.

The sub hit the bottom - 1575ft (480m) - at 09:30.

Mallinson says his first thought was relief they were alive. He later learned it crashed at 40 mph (65 kmh).

"We weren't injured but there was kit everywhere and we were hanging on to the pipe work. We just sat there with a torch. Unbeknown to us we had hit a gully, so we'd half disappeared below the seabed," says Chapman.

09:45 - Making contact

Pisces III made telephone contact, sending a message that they were both fine, morale was good and they were getting organised.

Early indications suggested oxygen supplies would last until early Saturday morning. The sub carried 72 hours of oxygen in case of an accident, but they'd already used eight hours on the dive. They had 66 hours left.

10:00-16:30 - Scrambling ships

The pilots spent the first few hours "getting sorted", according to Chapman. "The submersible was almost upside down, we had to rearrange it, mend the kit and make sure nothing was leaking," he says.

They decided if the oxygen was going to last, they needed to do very little. "If you switch off, you use one quarter of the oxygen. You don't talk or move," he says.

The two men lay as high up in the sub as possible above the foul, heavy air sitting at the bottom, according to Mallinson. The internal diameter of the crew sphere was just 6ft so the men had little space.

"We hardly spoke, just grabbing each other's hand and giving it a squeeze to show we were alright. It was very cold - we were wet through. I wasn't in the best condition anyway as I had just suffered three or four days food poisoning from a horrible meat and potato pie. But our job was to stay alive," he says.

Image caption Roger Chapman asleep in Pisces III

On the surface the rescue effort was under way.

Support ship Vickers Venturer, then in the North Sea, was contacted just after 10:30 and ordered to return its sister submersible Pisces II to the nearest port.

The Royal Navy's HMS Hecate was sent to the scene with special ropes at 12:09 and RAF Nimrod aircraft flew overhead.

A US Navy submersible, CURV III - designed to pick up bombs from the sea - was sent from California and Canadian Coast Guard ship John Cabot departed from Swansea.

Thursday 30 August

Mother ship Vickers Voyager arrived in Cork at 08:00 to load Pisces II and Pisces V, which had arrived overnight by aircraft. The ship sailed from Cork at 10:30.

Meanwhile, Chapman and Mallinson watched supplies begin to dwindle.

Image caption Chapman, now 68, had just got married at the time of the accident

The pair only had one cheese and chutney sandwich and one can of lemonade, but they didn't want to eat or drink them, according to Chapman.

"We allowed the CO2 to build up a bit to conserve oxygen - we had egg timers to keep track of every 40 minutes, but we'd wait a bit longer. It made us a bit lethargic and drowsy.

"We also started thinking about our families. I'd just got married, so could focus on my wife June. But Roger Mallinson had four young kids and a wife, and he began to get a tiny bit distressed about how they were," he says.

However Mallinson says one ship left the pilots a message which did "a hell of lot good".

"We got a message from Queen Elizabeth, who sent her best wishes for our quick recovery - which was a real warmer. It's amazing how bitterly cold you can feel, and then something happens that gets the adrenalin going, the heart rate up.

"It turned out it was the Queen Elizabeth II, which had altered its course from America to stand station with us in response to the mayday. But because it was so formal, we'd assumed it was the Queen. Then the message came 'sorry boys, wrong lady'."

Friday 31 August

"Friday was a disaster from a surface point of view," says Chapman.

First Pisces II was launched - with a special polypropylene rope attached to a "toggle" or collapsible snap hook - at 02:00, but the lifting rope tore from the manipulator because of its buoyancy, so it had to return to the mother ship for repairs.

CURV US Navy's CURV (cable-operated undersea recovery vehicle) was among earliest remote-controlled underwater vehicles

CURV-I grabbed public attention in 1966 when it was used to recover hydrogen bomb from bed of Mediterranean sea

CURV-III (pictured) also used to survey wreckage of SS Edmund Fitzgerald, tanker which sank in Lake Superior in 1975

CURV-III now superseded by CURV-21, which can be operated 20,000 ft (6.1km) underwater

Then Pisces V - launched again with a polypropylene line attached to a toggle - managed to make it to the seabed but couldn't find the stricken Pisces III before it ran out of power. It returned to the surface and later tried again.

"It was nearly 1pm before Pisces V found us. It was amazingly encouraging to know someone knew where we were. But when Pisces V tried to attach a snap hook the attempt failed because of the buoyancy of the rope," says Chapman.

Pisces V was ordered to stay with Pisces III, despite the fact it couldn't lift it. Pisces II descended again, but had to resurface after it got water in its own sphere.

Then CURV III - which had arrived with the John Cabot at about 17:30 - had an electrical fault so was unable to launch.

"By midnight on Friday we only had Pisces V out of almost everything, and two broken submersibles," says Chapman.

"Then Pisces V was ordered to the surface just after midnight, which was a bit of a blow. It was like we were back to square one with no-one around. Our 72 hours of oxygen was up, we were running out of lithium hydroxide to scrub the CO2, it was very manky and cold and we were almost resigned to thinking it wasn't going to happen."

Mallinson agrees that hope was fading.

He says one thing that helped him was the presence of dolphins. "We'd seen them on the 28th, and even though we couldn't see them, I could hear them on the underwater telephone for the entire three days. That gave me a lot of pleasure," he says.

Saturday 1 September 04:02 - Getting a line

Pisces II was launched again with a specially designed toggle and another polypropylene line.

"Just after 5am it had a line on us, on the aft sphere - they knew we were still alive," says Chapman.

"Then at 0940 CURV III came down and fixed another line, with the toggle inserted in the aft sphere opening. We were wondering what was going on, why we weren't being lifted."

Chapman says it was at this point - when the pilots knew the line was attached - that they had the can of lemonade and sandwich.

But Mallinson says he didn't feel confident the lift would work.

"The aft sphere wasn't the strong point - we were in the fore sphere, and I was very annoyed we weren't being lifted by that. I thought it was the wrong decision.

"I think at that point if they'd asked either of us if we wanted to be left or lifted we'd both have said 'leave us alone' - the recovery was so terrifying and the chances of getting up next to none," he says.

10:50 - The lift

Lifting of Pisces III started. "As soon as we got off the sea bed it was very rough, very disorientating," says Chapman.

The lift was stopped twice during ascent. Once at 350ft, for CURV to be disentangled, and a second time at 100ft, so that divers could attach heavier lift lines.

"We were thrashing and rocking about so they needed to get more ropes, so they could all be heaved together," says Mallinson.

13:17 - Clearing the water

Image caption Chapman and Mallinson after being brought to the surface

Pisces III was dragged clear of the water.

"Apparently they thought we'd died when they looked at us, it had been so violent," says Chapman.

"When they opened the hatch and fresh air and sunlight rushed in it gave us blinding headaches, but we were sorted, we were euphoric. But we were also a bit pathetic. It was quite difficult to climb out of the sub, we'd been so cramped up, we could hardly move."

In fact Mallinson says it took a good 30 minutes to open the hatch. "It had been jammed shut and wouldn't open upside down. When it did open, it went off like a gun, we could just smell salty sea air," he says.

The pilots had been in Pisces III for 84 hours and 30 minutes when they were finally rescued.

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

The aftermath

The rescue attempt captured the imagination of the media and the public.

Shortly after the rescue, Roger Chapman left Vickers and formed the company Rumic, providing subsea services and operations to the offshore and defence industries.

He has become a leading authority on rescue submersibles, being mobilised to the sinking of the Kursk on behalf of the Royal Navy in 2000, and playing a central role in successfully rescuing the seven-man crew of Russian submarine AS-28 Priz in 2005.

Rumic was acquired by James Fisher and Sons the following year, and is now known as James Fisher Defence.

Meanwhile Mallinson, 75, who lives in the Lake District, carried on working for the same company in submersibles until 1978.

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He's now heavily involved in restoring steam engines, receiving a Lifetime Achievement award from Prince Michael of Kent for his involvement with The Shamrock Trust, in Windermere, in 2013.

The two men still keep in touch, meeting up every year.

While Chapman's brush with death has clearly influenced his career, the 68-year-old says it hasn't had many other repercussions.

"I'm a bit more reluctant to go in a lift - I think it's the up and down - but that's the only thing that physically worries me," he says.

Mallinson says if the submersible went down again, he "wouldn't do anything differently".

"Roger Chapman is a great lad. Somebody else might have panicked. If I could have chosen anyone to go down with it would have been him," he says.

Now their ordeal is set to be made into a film.

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