The depth charge
A depth charge explodes astern of HMS Starling. Image copyright Imperial War Museum, reference IWM A22031
Depth charges consisted of steel drums filled with high explosives. They could be set to detonate at different depths. They worked on the principle that water pressure increases with depth. An inlet valve controlled the depth at which the charge exploded. The water drove a detonator against a primer, which would explode and set off the main charge.
Charges could be rolled over a ship's stern, or fired from mortars on either side of a vessel. In time, heavy weights were bolted to them, causing them to sink faster and explode deeper. An exploding charge could destroy a U-boat twenty-five feet away and damage one fifty feet away. They also often produced trauma similar to shellshock in U-boat crews.