Only two nuclear powered US submarines have been unintentionally sunk. The other is the USS Scorpion (SSN-589).
She was the first ship of a new class of submarine and was commissioned in 1961 after being built in Kittery, Maine, at the Portsmouth Naval Shipyard. Being the first ship of the class, her first two years were spent undergoing various tests and sea trials in the Caribbean and Atlantic.
As is normal, she was dry docked for overhaul following the initial trials, the crew refreshed, and sent out again for a second set of sea trials in 1963.
As air expands it cools, any moisture contained in the air or near the inlet port may be cooled below its freezing point. While undergoing emergency ballasting drills it is believed that the valves to the ballast tanks became ice bound, limiting the flow of air into the tanks below the level required to allow the ship to achieve the bouyancy necessary to surface.
Since it was on sea trials, there was a continuous radio link with a rescue/tender ship on the surface. The Thresher sank to crush depth and all hands were lost on April 10th 1963