129 Lost Beneath the Sea The Day a Submarine Imploded, April 10, 1963 — The USS Thresher, the pride of the U.S. Navy’s new generation of nuclear submarines, slipped beneath the Atlantic for a deep-diving test off the coast of Massachusetts. She was one of the most advanced subs of her time — fast, silent, and deadly — capable of tracking enemy vessels with unmatched precision. But this test dive would be her last. At a depth of over *1,300 feet*, communication with Thresher suddenly went silent.