In March 1968, Soviet submarine K-129 sank 1,560 miles northwest of Hawaii. The sub carried more than 70 crewman, none of whom survived. It also held several 4-megaton nuclear warheads and a slew of Soviet military intelligence.

The Soviets spent months looking for it, to no avail.

For the United States, it was a treasure too valuable to resist. When American sonar picked up the wreck of K-129, officials launched a secret mission involving the CIA, billionaire Howard Hughes and a mystery ship that cost more than $350 million.

The code name: Project Azorian.

The United States had tracked the sub at 16,500 feet below the surface, more than three miles down. Given its size and the depth involved, a successful recovery operation would have been the first of its kind.

In October 1970, a task force of engineers concluded that the best way to retrieve the sub, which was believed to have weighed 2,000 to 2,200 long tons (a long ton is 2,240 pounds), would be to construct a ship from scratch capable of lifting the sub straight off the ocean floor. (Other options were considered, including using gas to raise the object.)

The mission created two dilemmas: how to build a ship strong enough, and what to tell the public.

The agency found both answers in Hughes.

The government enlisted his shipbuilding operation to construct what would be known to the public as a deep-sea mining ship, the Hughes Glomar Explorer.

The ship was introduced to the public in November 1972 with a press announcement and “the usual champagne christening ceremony,” according to CIA documents declassified decades later.

The whole thing was a ruse. The 600-foot-long ship was really built specifically for one mission, with “ingenious devices straight from a Bond film,” according to the BBC.

“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.”

Popular Mechanics reported, “Camera systems, sonar and hydraulics all had to be designed on spec.

“The government had to turn to the best optics people in the USA to figure out how to image way down deep without creating distortions or shadows that would ruin this delicate work.”

Anticipating the recovery of the deceased Soviet sailors, the ship also had “refrigeration capacity for up to 100 bodies, and copies of the relevant Soviet and American burial manuals,” according to The New York Times.

Once constructed, the Hughes Glomar Explorer took off from Bermuda on Aug. 11, 1973, en route for a 12,700-mile, 50-day voyage to Long Beach, Calif., for intelligence upgrades.

The participation in the mission of two Chilean pilots caused some drama when the ship docked in Valparaiso, Chile, on Sept. 12, the day after the country experienced a coup d’état.

Representatives from Hughes’ company were already there, prepared to meet the ship with supplies and new personnel, when the coup occurred at 6 a.m. that Sept. 11.

“The Americans were awakened by noise outside the hotel,” the CIA documents recount. “It was evident the revolution had started as there were soldiers, tanks, armored cars and other military vehicles all over the city. The hotel was surrounded, communications cut off, and guests confined to the hotel for the next two or three days.”

While the report doesn’t go into further details, it indicates that a company representative, whose trip report read “like a Hollywood script,” managed to get all the supplies and personnel aboard the HGE in time to sail off on Sept. 13.

“The presence of a covert US intelligence ship in a Chilean port during the military coup was a bizarre coincidence quite unrelated to the rumors that ‘the CIA had 200 agents in Chile for the sole purpose of ousting [Chilean president Salvador] Allende,’ ” according to the CIA.

The ship arrived in Long Beach, Calif., on Sept. 30, and remained there until June 20, 1974, when it sailed off — complete with another public ceremony for “cover purposes” — and arrived at the recovery site on July 4.

On July 17, the Soviet naval ship the Chazhma, which had a helicopter onboard, was detected en route to the recovery site, expected to arrive the following day. The mission director “ordered that piles of canvas-covered crates be placed on the HGE’s helicopter deck to preclude the possibility that the Soviet helicopter might land on the HGE for any reason.”

The next day, the ship came within a mile of the HGE. Soviet personnel aboard the Chazhma began taking photos of the HGE, and its helicopter took off and did the same, with those inside “taking photographs from all angles.” The mission director aboard the US ship, meanwhile, “sent a number of crew members to the bow of the HGE to preclude any attempt by the Soviet helicopter to hover and lower personnel onto the bow.”

The helicopter returned to its ship after about a half-hour, but now the Americans were concerned that the Soviets had learned the true mission of the HGE. A decision was made that if the Soviets attempted to board the ship, any sensitive intelligence material aboard the HGE would be destroyed.

The Soviets sent the helicopter for one more round of photos, and sent a message asking the ship’s purpose. After the Americans indicated the HGE was a mining ship, the Soviet ship finally sailed away, signing off with the message, “I wish you all the best.”

The K-129 was within range by around July 28. According to the documents, there was a frightening equipment failure aboard the HGE that day. Most of the details are redacted, but the report notes there were “sparks, smoke, and spastic shaking of the derrick” that left the crew “very nervous . . . about safety.” It also noted, though, that the “no insurmountable damage was suffered.”

The crew began lifting the sub on Aug. 1, experiencing numerous problems with the heavy-lift system and coming close to aborting the mission several times.

The HGE got the K-129 about halfway up before the immense weight broke the grabbing crane, snapping the sub into pieces, with around two-thirds of it plummeting back to the ocean floor.

It was never revealed how much intelligence was recovered from what remained, the front portion of the ship. The crew did find the bodies of several Soviet submariners, and buried them at sea in a formal ceremony.

While the US first concealed any details of Azorian from the Soviets, the American government’s attitude changed when it sought information on missing US servicemen in Vietnam.

In an effort to secure Russian cooperation in this area in 1992, Robert Gates, then director of the CIA, met with Russian President Boris Yeltsin that October.

“As a gesture of intent, a symbol of a new era, I carried with me the Soviet naval flag that had shrouded the coffins of the half-dozen Soviet sailors whose remains the Glomar Explorer had recovered,” Gates wrote in his 1996 memoir, “From The Shadows.”

“I also was taking to Yeltsin a videotape of their burial at sea, complete with prayers for the dead and the Soviet national anthem — a dignified and respectful service even at the height of the Cold War.”

Another recovery operation, to bring up the rest of the ship, was considered in 1975. But news of the mission had become public by then thanks to a 1974 burglary of Hughes’ headquarters, where the stolen material included documents about Project Azorian.

To everyone’s surprise, the Soviets never issued a response.