Newly declassified secret documents reveal a deep-sea photoreconnaissance recovery mission 40 years ago at 16,400 feet below the Pacific Ocean near Hawaii.
The film images were sent back to Earth in recoverable return capsules. Entering the Earth's atmosphere, the canisters deployed a parachute and were then snagged by a plane in mid-air and returned to base for processing and analysis.
But in July 1971, the third reentry vehicle from the first Hexagon photo-satellite mission was lost, when the parachute broke, sending the canister into the open sea near Hawaii. The bucket sank on impact to a depth of more than 16,400 feet. This was sensitive info -- photographs of the Soviet Union's submarine bases and missile silos -- and the decision was made to attempt to recover the valuable intelligence data.
This week, the CIA released documents relating to the spy satellite incident and the recovery mission. Here's an illustration of the Hexagon system.