Operation Royal Marine was a military operation in World War II, in which naval mines were floated down rivers from France into Germany to destroy bridges and shipping. Promoted by Winston Churchill, it was planned in November 1939. However, Sir Edward Spears claimed that he had originally proposed the idea to Churchill when they visited eastern France in August 1939, but by the time the operation was put into practice Churchill believed the idea was his. Trials of the mines were carried out in the Thames in December 1939. Each mine contained 15 pounds (6.8 kilograms) of TNT and, depending upon type, either floated or bounced along the riverbed. The mines were especially developed for the operation by MD1.