On April 9th, 1963, the nuclear-powered submarine USS Thresher departed Portsmouth Naval Shipyard for depth tests in the Atlantic. It would never return.

Videos:

• Second Saturday: The Loss of the USS Thresher
• Finding the Titanic | Bob Ballard: An Explorer's Life

Articles and books:

• U.S. Naval Institute: What Killed the Thresher?
• Arlington National Cemetery: USS Thresher memorial
• Titanic Discovery was By-Product of Military Quest
• The Death of the USS Thresher: The Story Behind History's Deadliest Submarine Disaster, by Norman Polmar
• Navy documents reveal officer flagged "dangerous condition" before deadliest submarine disaster in U.S. history