Given that there’s such little margin for error in a submersible, many submarine losses remain sources of intrigue and mystery, and few rival the disappearance of the Soviet submarine K-129 in 1968. The Cold War was in high gear, and under the waters of the world’s oceans, Soviet and American submarines were in intense competition, pushing the boundaries of new technologies. Some of these submarines carried nuclear missiles that carried more destructive power than all the bombs dropped by the U.S. Army Air Force throughout World War II, including the nuclear bombs dropped on Hiroshima and Nagasaki. Other submarines, the hunter-killers, were designed to find and destroy these missile-carrying submarines. All of these submarines engaged in their activities while aiming to avoid detection by the other side, establishing a clandestine conflict that was carried out far from the gaze of the public but still strategically vital. If either side could gain a notable advantage, it could abruptly change the fragile balance of power.

Both sides lost submarines in the 1960s, but when the Soviets lost K-129 in 1968, it triggered one of the most interesting episodes of the Cold War, and something that reads like the plot of a techno-thriller. A race developed between the Americans and Soviets to locate the wreck, and when the Americans found it, they realized the submarine sunk in one of the deepest parts of the ocean, far beyond the depths at which conventional divers or submarines could operate. As a result, the CIA, aided by eccentric billionaire Howard Hughes, clandestinely began a lengthy and incredibly expensive project to build equipment that might allow them to recover and examine parts of the sunken submarine.

That operation, Project Azarian, is still controversial over 50 years later.