At the start of World War II, the depth charge was the only weapon available to a vessel for destroying a submerged submarine. Depth charges were dropped over the stern and thrown to the side of a warship travelling at speed. Early models of ASDIC/Sonar searched only ahead, astern and to the sides of the anti-submarine vessel that was using it: there was no downward-looking capability. So there was a time lag between the last fix obtained on the submarine and the warship reaching a point above that position. Then the depth charges had to sink to the depth at which they were set to explode. During those two delays, a capable submarine commander would manoeuvre rapidly to a different position and avoid the attack. The depth charges then left an area of disturbed water, through which it was difficult to regain ASDIC/Sonar contact. In response to this problem, one of the solutions developed by the Royal Navy was the ahead-throwing anti-submarine weapon - the first of which was Hedgehog.