During World War Two, Germany became the only country in history to operate submarine tankers, a circumstance forced by the need to refuel U boats in distant operational areas reached only by passing through large areas of Allied-controlled ocean. Conventional surface tankers had been too easy to locate by Allied aircraft and naval patrols, but the submarine tankers were undetectable once submerged and sailing towards their secret refuelling rendezvous. The submarine tankers, known affectionately as 'milk cows', gave the attack U boats the ability to double or sometimes even treble active operational time in their patrol area. Thus both Germans and Allies regarded the cows as the most important units of the U boat fleet, and Allied forces had explicit orders to attack the tankers first, whenever a choice existed between U boat targets.