The underwater protection was an internal "bulge" that consists of four longitudinal torpedo bulkheads forming a multi-layered system designed to absorb the energy from an underwater explosion equivalent to 700 pounds of TNT (1.3 GJ). The protection system was designed for the torpedo bulkheads to deform and absorb energy while several compartments were liquid loaded in order to disrupt the gas bubble; the total system depth was . It was hoped that extending the armor belt to the triple bottom where it tapers to as one of the torpedo bulkheads can help increase the protection of the system compared to the one used by the previous "North Carolina"-class. However, caisson tests in 1939 indicated that the initial design of the newer system was "less" effective due to the rigidity of the armor belt causing leakage into adjacent compartments, particularly at the welded joint between the lower armor belt and the triple bottom. This resulted in several modifications, including reinforcement of the lower belt armor and triple bottom joint with buttstraps and a subsequent alteration of the system's liquid loading scheme; whereas the "North Carolina"s had the third and fourth outboard compartments liquid loaded, the "South Dakota"s had the outer two compartments filled with liquid, typically fuel oil, while the inner two compartments were void spaces; this was done to mitigate the problematic effects of the lower armor belt's rigidity.