The Allies' answer: the depth charge. These weapons were developed under the orders of Adm. Jellicoe at the demand of his captains in the North Atlantic for a sort of portable mine. Metal oil drums filled with 300 lbs of TNT (136 kg) detonated by a hydrostatic pistol, depth charges were developed as a desperation measure and first deployed widely at the end of 1916 -- a crude, inaccurate, but terrifying weapon. The hydrostatic pistol (2) is in the charge's axis to the left. Water entering a bellows chamber (1) forced the plunger down until it met the primer (3) at a preset depth. Kaboom! The weapon's effectiveness was enhanced by the deployment of a thrower or "Y" gun in 1918. In WWII a pattern of smaller charges could be thrown automatically from a device known as a Hedgehog.
Rendering by John Batchelor; copyright © 1979 by Time-Life Books - The Seafarers.