Torpedo defence
Torpedo defence includes evasive maneuvers, passive defense like torpedo belts, torpedo nets, torpedo bulges, and sonar torpedo sensors, "soft-kill" active countermeasures like sonar decoys and sonar jammers, and "hard-kill" active defenses, like anti-torpedo torpedoes similar in idea to missile defense systems. Surface Ship Torpedo Defense and Countermeasure Anti-Torpedo systems are highly experimental and the US Navy ended trials on them in 2018.