World War I marked the first earnest conflict involving significant use of submarines and consequently marked the beginning of major efforts to counter that threat. In particular, the United Kingdom was desperate to defeat the U-boat threat against British merchant shipping. When the bombs that it employed were found to be ineffective it began equipping its destroyers with simple depth charges that could be dropped into the water around a suspected submarine's location. During this period it was found that explosions of these charges were more efficient if the charges were set to explode below or above the submarine. However, many other techniques were used, including minefields, barrages and Q-ships and the use of cryptanalysis against intercepted radio messages. The airship ("blimp") was used to drop bombs but fixed-wing aircraft were mostly used for reconnaissance. However, the most effective countermeasure was the convoy. In 1918 U-boat losses became unbearably high. During the war a total of 178 U-boats were sunk, by the following causes: