New Britain Submarine Disappears For 23 Hours With 79 Men On Board During Trials: Destroyers speeding to the scene of the disaster - picture from the air. With 79 men on board, Britain's newest submarine the "Thetis", disappeared for 23 hours off Great Orme's Head, North Wales. She was undergoing acceptance trials after leaving the Cammell Laird at Birkenhead, Cheshire, and had on board in addition to her crew of 23 a large number of experts. The "Thetis" dived in 130-feet of water and failed to return to the surface, but after 23 hours she was seen partly above water and men on board started to leave by means of the Davis escape apparatus with which all British submarine are equipped. June 02, 1939.