From the 1970s on, the US Navy hasn't lost a single submarine. But in April 1963 it suffered its deadliest submarine disaster when the nuclear-powered USS Thresher sank off Cape Cod, killing all 129 people on board. The details of what led to that fate had been kept under wraps for decades, until retired US sub commander James Bryant sued the Navy for the release of accident investigation files. In 2020 he won, and Popular Mechanics reports on what the declassified documents reveal: chiefly, that there wasn't one root cause, at least in Bryant's eyes. The Thresher, the lead boat in its class of nuclear-powered attack subs, was meant to go head-to-head with the Soviet Union's own new class of nuclear subs. It was doing deep dive tests on the fateful morning when it alerted monitoring ships on the surface that it was experiencing "minor difficulties" and would resurface.
It never did, and the AP reported sailors on the ships above listened as "the sub disintegrated under the crushing pressure of the sea." It was later found in pieces on the Atlantic seafloor. The Navy's investigation pointed the finger at a seawater pipe with faulty welding whose failure resulted in a leak that shorted out the Thresher's electrical system. But Bryant and others who have reviewed the files think there were multiple factors at play, including a rush to counter the Soviets that resulted in crews who weren't as well trained and failed to react quickly enough to save the ship, as well as a false belief that nuclear-powered submarines could not fully lose power, reports USNI News. As for why the files have been kept under wraps, Bryant says what he has read makes clear that it wasn't due to a coverup, but rather to keep operational details out of Soviet hands. (Read more nuclear-powered submarine stories.)