In the 1930s, Germany had experimented with the laying of mines by aircraft. It became a crucial element in their overall mining strategy. Aircraft had the advantage of speed, and they would never get caught in their own minefields. German mines held a large explosive charge. From April to June 1940, the Luftwaffe laid 1,000 mines in British waters. Soviet ports were mined, as was the Arctic convoy route to Murmansk. The Heinkel He 115 could carry two medium or one large mine while the Heinkel He 59, Dornier Do 18, Junkers Ju 88 and Heinkel He 111 could carry more.