On the morning of May 23, 1939, the U.S.S. Squalus sunk. The brand-new, state-of-the-art submarine was conducting a routine test dive off the coast of New Hampshire when an induction valve failed. Water rushed into the submarine's engine room as well as the crew's cabin. Twenty-six men immediately drowned. When the Squalus' sister submarine located the sunken vessel, 33 men were still alive and able to communicate via Morse code. "Conditions satisfactory but cold," telegraphed Squalus' commander, Lieut. Oliver F. Naquin.
Navy divers led by Charles Momsen — inventor of an escape breathing apparatus called the Momsen Lung — used a rescue bell, a small vessel tethered to a ship that could be submerged and attached to the sub's escape hatch. It took four separate rescue trips, but all 33 men were brought to shore safely. The final survivors reached the surface just after midnight on May 25.