With the discovery that depth charges rarely scored a kill by hitting a submarine, but instead were most effective in barrages, it was found that similar or better effects could be obtained by larger numbers of smaller explosions. The anti-submarine mortar is actually an array of spigot mortars, designed to fire off a number of small explosives simultaneously and create an array of explosions around a submarine's position. These were often called "Hedgehogs" after the name given to a World War II British design. Later ASW mortar shells were fitted with impact detonators that fired only after actual contact with the hull of the submarine, allowing sonar crews to maintain a constant sound track until a hit was achieved.