In the summer of 1974, a large and highly unusual ship set sail from Long Beach in California.

It was heading for the middle of the Pacific where its owners boasted it would herald a revolutionary new industry beneath the waves.

Equipped with a towering rig and the latest in drilling gear, the vessel was designed to reach down through the deep, dark waters to a source of incredible wealth lying on the ocean floor.

It was billed as the boldest step so far in a long-held dream of opening a new frontier in mining, one that would see valuable metals extracted from the rocks of the seabed.

But amid all the excited public relations, there was one small hitch - the whole expedition was a lie.

This was a Cold War deception on a staggering scale, but one which also left a legacy that has profound implications nearly half a century later.

The real target of the crew on board this giant ship was a lost Soviet submarine. Six years earlier, the K-129 had sunk 1,500 miles north-west of Hawaii while carrying ballistic nuclear missiles.

The Russians failed to find their sub despite a massive search, but an American network of underwater listening posts had detected the noise of an explosion that eventually led US teams to the wreck.

It was lying three miles down, deeper than any previous salvage operation. The weapons and top-secret code books were surely beyond reach.

But in the struggle for military advantage, the sub represented the crown jewels – a chance to explore Moscow’s nuclear missiles and to break into its naval communications.

So the CIA hatched an audacious plan, Project Azorian, to retrieve the submarine. That would have been hard enough. But there was another challenge as well - it had to be done without the Russians knowing.

The spies needed to create a smokescreen so they pretended to be exploring the possibility of deep sea mining.

A PR campaign conveyed a determined effort to find manganese nodules. These potato-sized rocks lie scattered in the abyss, the great plains of the deep ocean.

There had to be a frontman - someone rich and eccentric enough to be plausible. The reclusive billionaire inventor Howard Hughes was perfect for the role.

He agreed to take part and, in his name, a unique ship was designed. Publicly, it was fitted with everything needed to dig up the seabed.

But, covertly, the Hughes Glomar Explorer was also built with ingenious devices straight from a Bond film. The ship’s hull had enormous doors that could swing apart to create a “moon pool”, an underwater opening large enough to accommodate the Soviet sub and keep it hidden.

Tucked away out of sight inside the ship was a “capture vehicle” which had a giant set of claws to straddle the sub and secure it.

It took until 1974, six years after the sinking of the sub, for the CIA to be ready. The cost of the project - $500m - was equivalent then to building a couple of aircraft carriers or launching an Apollo mission to the moon.

No-one had ever attempted anything on this scale in such incredible depths. The sub itself had a weight of nearly 2,000 tonnes but the three miles of thick steel pipe needed to haul it up added even more.

New systems were needed to keep the Glomar Explorer in position as well as to handle the huge load, and everyone on board was nervous. Dave Sharp, one of the few CIA figures happy to talk about the project, tells me it was “really frightening” when heavy seas threatened to tear their unusual vessel apart.

But even more alarming was the suspicion of the Russians. To convince them that Howard Hughes was genuinely interested in nodules, executives were despatched to conferences on ocean mining where they described in detail their plans to harvest the rocks.

“We made ocean mining seem a lot more credible,” Sharp says. “We really misled a lot of people and it’s surprising that the story held together for so long.”

The cover was so good that it prompted US universities to move to start courses in deep sea mining and it also whipped up the share prices of the companies involved. “People thought, ‘if Howard Hughes is into it, we need to be too’,” says Sharp.

“We even collected a few nodules,” he remembers, which was fortunate because Soviet spy ships kept a constant vigil and once even came close enough to overhear the Americans’ conversations.

“When we realised they were right alongside, we started talking about nodules, like ‘here’s a good one’ so it looked like we were checking them.”

Yet another complication arose. The project needed calm weather and that was only likely in summer. But just when it was about to begin in summer 1974, US President Richard Nixon was visiting Moscow for a peace-making summit.

Being caught stealing a Soviet sub would not exactly have helped, so Nixon insisted that the operation could not begin until he had left Russia. That was on 3 July. By then the Hughes Glomar Explorer was in position and the winches whirred into action the next day.

Things did not go smoothly. Sharp recalls that pumps and connections kept breaking. Huge vibrations rocked the ship as the “capture vehicle” was “banging back and forth in the waves”. But on 30 July, he watched as underwater cameras relayed video of the sub as well as “dozens of crawling crab-like crustaceans” and a big white fish that looked like a shark.

Amazingly, the giant steel claws successfully seized the sub. But then disaster struck. At some point on the way up, the immense strain became too much, part of a claw snapped off and most of the sub slipped back to the seabed.

Only the front section made it up. The bodies of six Soviet submariners were recovered and were later given a formal burial at sea. But the missiles and code books were never found.

The CIA official history asserts that the operation was one of the greatest intelligence coups of the Cold War, but it had cost vast sums and questions immediately arose about its value. A year later, the sensational details became public and plans to recover the remaining section were abandoned.

As Sharp puts it, the revelation that the deep sea mining project was fake was “a sudden shock” to other mining companies and also to diplomats at the UN who were right in the middle of negotiating future rights to ocean minerals. Share prices tumbled amid a wave of recriminations.

This might have derailed the very notion of deep sea mining for good. But in fact it proved that with clever engineering and a lavish budget it was possible – just - to operate in the otherworldly depths. “It’s really difficult but we showed it could be done,” says Sharp.