Why a modern battleship cannot shrug off torpedoes

How could a 21st-century battleship survive a spread of Mark 48 torpedoes?

It probably could not; reliable under-keel detonations bypass the protection logic used against older side-impact torpedoes.

Traditional anti-torpedo protection addressed explosions against a ship's side. Modern heavyweight torpedoes can detonate reliably beneath the keel, defeating that protection's basic assumptions. Acoustic homing lets a torpedo pursue a maneuvering ship, and attack settings can direct it toward the bow, propulsion machinery, or amidships. A battleship could not count on absorbing a full spread of Mark 48s, and even a modern nuclear-powered carrier might not survive such an attack. Decoys such as Nixie offer a way to divert the torpedo before it reaches the ship.