In late 1971, the Navy brought the deep-diving submersible Trieste II and several support ships out to a spot 350 miles north of Hawaii to retrieve Scripps Institution of Oceanography "instrumentation" from the ocean floor.

That was the cover story.

The Trieste actually was tasked by the Central Intelligence Agency with a secret mission to recover a film canister jettisoned on July 10 of that year by a spy spacecraft known as Hexagon.

Two film canisters, called "buckets," that had been successfully recovered over the prior month provided images of Soviet missile sites and other sensitive military assets as the Cold War raged.

The third canister should have been snagged by an Air Force C-130 aircraft as the film payload floated to Earth beneath a parachute.

Instead, the parachute broke from the canister, and the 1,051-pound bucket hit the water at about 307 mph, sinking to a depth of 16,400 feet.

What followed was the deepest underwater salvage operation ever mounted by the U.S. military amid an urgency to recover and examine the film and as worries were raised that the Soviets might catch on or get there first.

"Recovery of the film would be most desireable since the imagery recorded was from a particularly productive portion of the mission," John L. McLucas, then-director of the National Reconnaissance Office, said in a 1971 memo.

Details of the recovery mission, declassified during the summer, shed new light on the successful series of satellite spy programs called Corona, Gambit and Hexagon that used Hickam Air Force Base crews and aircraft to recover surveillance footage shot from space.

Some of the Hexagon film was recovered from the seafloor but was unusable, and the salvage record would be eclipsed in 1974 by the Glomar Explorer’s efforts to raise the Soviet submarine K-129 from nearly 17,000 feet 1,500 miles northwest of Hawaii.

But the Hexagon mission tested new operating depths for the Trieste II and "established a unique capability that enhances the security of the United States," Harold L. Brownman, who was with the CIA, said in a secret May 1972 memo lauding the Navy effort.

"This was the first time that an object of this small size had been located and the first time any object had been recovered from this depth," Brownman said.

After World War II, the United States developed new photo capabilities to penetrate "denied areas" in the Soviet Union, Eastern Europe and Asia, according to the National Reconnaissance Office.

President Dwight D. Eisenhower directed the CIA to develop, in conjunction with the Air Force, the nation’s first photoreconnaissance satellite, code-named Corona.

First launched in 1960, Corona operated with much less risk than aircraft and searched broad areas to capture valuable imagery while orbiting high above the Earth, the NRO said.

Corona was declassified in 1995. Declassification of the follow-on Gambit and Hexagon programs was announced on Sept. 17, 2011.

Hexagon first launched in June 1971 and was a primary method of verifying Soviet compliance with strategic arms limitations, according to the NRO.

The Hexagon spacecraft "was as big as a locomotive," carried 60 miles of film, and had four film buckets, or recovery vehicles, the NRO said. Launched atop a Titan III-D, it was nicknamed "Big Bird."

On June 20, 1971, the first film bucket re-entered the Earth’s atmosphere in the Hawaii recovery area with a badly damaged parachute, but crews got to it before it sank.

The second and fourth buckets were snatched out of the sky, but the third plummeted to the sea on July 10 and disappeared beneath the waves.

"In view of the importance of the intelligence collection" on the lost recovery vehicle, the Navy was asked to retrieve it, documents state. The Eastman Kodak Co. was consulted and said the imagery likely would be salvageable.

The USNS De Steiger found the payload in October, while the dock ship White Sands and fleet tug Apache were used to support the Trieste II.

Crew members were told the cover story that the Trieste II needed to retrieve Scripps Marine Physical Laboratory equipment that was light-sensitive, hence requiring a black shroud over the submerged equipment and night recovery.

Planners fabricated a "hay hook" claw to snare the sunken spy film and recovery was practiced off San Diego.

The White Sands, Trieste II and Apache arrived at the search area Nov. 1, but problems locating the film, submersible malfunctions and heavy seas led to at least two failed retrieval attempts and a return to Pearl Harbor.

In August 1971, a month after the film payload was lost at sea, NRO staff director Col. David Bradburn sent a top-secret cable to Brownman at the CIA asking if the Soviets knew or suspected that the U.S. had lost a spy satellite payload and if they had the capability to recover it.

Brownman wrote back saying that "we have no evidence that the Soviets know or suspect that we failed to recover" the film bucket.

"We are virtually certain they have no capability to recover it," he said.

Brownman said the Soviets did keep a seagoing tug on station in that part of the Pacific, primarily to support their nuclear submarines, and that it did have sophisticated communications and conceivably could monitor Hexagon film capsule re-entries.

On Jan. 7, 1972, Hexagon program manager Col. Frank Buzard sent a message to the NRO expressing concern that the Soviets had ships with manipulator arms that possibly were capable of reaching depths of 33,000 feet.

"Now that we have established a precise area of extreme interest, which the Soviets must be aware, we should not abandon the site until our objective is achieved," Buzard said, raising the possibility that the Soviets could move in.

The Navy team was turned back by bad weather and other setbacks through November 1971. Finally, on April 25, 1972, the film was in the grasp of the Trieste II, which was then instructed to proceed — slowly — to the surface.

"The Trieste surfaced and immediately reported that the payload had disintegrated in a cloud of silt with a few pieces falling through the tines of the hook," said a memorandum for the record. "Everyone went from an emotional high to an emotional low in about one microsecond. Divers were put into the water and managed to retrieve some of the film."

The film stacks had been recovered intact, but disintegrated as they were brought to the surface. The imagery shot from space had been lost.