The Balao class is a design of United States Navy submarines that was used during World War II, and with 120 boats completed, the largest class of submarines in the Navy. An improvement on the earlier Gato class, the boats had slight internal differences. The most significant improvement was the use of thicker, higher yield-strength steel in the pressure hull skins and frames, which increased their test depth to 400 feet (120 m). A Balao-class submarine, the USS Tang, actually exceeded her depth gauge's maximum reading of 612 ft (187 m) in 1944, when taking on water in the forward torpedo room while evading a destroyer.