This work surveys the emergence of underwater warfare during the Great European War, combining technical explanation with operational narrative. It reviews modern submarine design and armament, profiles the submarine fleets of the principal naval powers and recounts early actions, then outlines torpedo and mine technology, mine-laying and sweeping practices, harbour defences and anti-submarine tactics. Chapters describe submarines in action and assess comparative fighting value of national flotillas, presenting how submarines, mines and torpedoes reshaped naval strategy, convoy protection and the defence of harbours and sea lanes.