The USS Thresher (SSN-593), which sank 60 years ago this April, was the world’s first nuclear-powered submarine to be lost at sea. Amid the public shock over the tragedy, the U.S. Navy grappled for an answer as to what went wrong. Even today, rival theories seek to explain the mystery.

The Thresher got under way on her initial sea trials from the Portsmouth Naval Shipyard in Kittery, Maine, on 30 April 1961. She was the first of a new class of nuclear-powered submarines, combining the attributes of an “attack” submarine and a specialized “hunter-killer” craft. The submarine had a cigar-shaped hull that was derived from the research submarine Albacore (AGSS-569) for efficient underwater performance. In her bow was the massive BQQ-2 sonar, heralded as the most advanced sonar ever fitted to a submarine, being credited with unprecedented passive detection ranges. The sonar “dome” was 15 feet in diameter mounting 1,241 transducers.