A principal problem for invasion planners is supply, particularly of fuel for a large mechanised military force. Surface transportation can be slow, unreliable and risky. So Operation Pluto (Pipe-Lines Under The Ocean) was a Second World War operation by British engineers, oil companies and armed forces to construct secure undersea oil pipelines beneath the English Channel between England and France in support of Operation Overlord, the Allied invasion of Normandy in June 1944.

More Info: en.wikipedia.org