Given that there’s such little margin for error in a submersible, many submarine losses remain sources of intrigue and mystery, but in American history, few rival the tragedy and drama aboard the Squalus in 1939. On May 23 of that year, the Squalus, one of the U.S. Navy’s newest submarines, was undertaking the latest in a series of test dives off the coast of New Hampshire. She had been commissioned less than three months before and represented the cutting-edge of new submarine technology. Provided with eight torpedo tubes and a three-inch deck gun, Squalus had a range of over 11,000 miles and was capable of running at over 20 knots on the surface. This was a formidable warship, and the test dives undertaken in May were a routine part of preparing the submarine for service.

After successfully completing a series of dives over the previous days, on the 23rd, the Squalus undertook practice for one of the most challenging maneuvers, a rapid dive performed while the sub was running at full speed on the surface. This was a vital procedure if the submarine was attacked unexpectedly by an aircraft or surface ship - it had to be able to dive quickly in any circumstances. This practice ensured that the crew, and the submarine, were fully prepared.

Tragically, during this test dive, the unthinkable happened: the rear section of Squalus flooded, and the submarine, without power, began to sink to the bottom of the Atlantic Ocean. 26 men in the rear compartments died in the flooding, but 33 men in the forward compartments were still alive as Squalus came to rest on the ocean floor almost 250 feet below the surface.

The situation seemed hopeless to everyone but one naval officer, Lieutenant Commander Charles B. "Swede" Momsen.