Essentially, the bulge is a lateral compartment at the waterline level that is isolated from the ship's internal volume. It is part air-filled, and part free-flooding. In theory, a torpedo strike will rupture and flood the outer air-filled component of the bulge and the inner water-filled part will dissipate the shock and absorb explosive fragments, leaving the ship's main hull structurally intact. Transverse bulkheads within the bulge limit flooding throughout the entirety of the structure. Anti-torpedo bulges were still added as retrofits to older ships, such as HMS Renown and USS Arizona.