Momsen, already famous for the invention of his Momsen lung, achieved even more fame for directing the rescue and recovery of the 33 crewmen of the submarine "Squalus", which sank in May 1939 in of water off the Isles of Shoals, New Hampshire. Working from the submarine rescue ship USS "Falcon" (ASR-2), Momsen instructed the team of deep-sea divers as they dived to the submarine and attached cables to the rescue chamber. He also supervised rescue chamber operators as it made four dives to bring the submariners to the surface and a fifth to check the flooded aft section for survivors. The fourth dive was marred by a cable jam, and the chamber had to be hauled to the surface by hand over hand pulling by all on board. All 33 surviving crewmen were rescued. 26 men had perished.