It was not infrequent for maritime patrol aircraft, during both world wars, to misidentify whales as submarines. From the air, when submerged, both whales and submarines look like large dark shapes, and they are easily confused. When surfacing, whales also made similar shapes on the surface of the water, adding to the likelihood of confusion. This led to a considerable number of attacks on whales by aircraft, mainly Allied ASW patrol aircraft during WWII. American units would give pilots who bombed whales by mistake a satirical medal, the 'Royal Order of the Whale Bangers'. Aircraft radar was less likely to detect whales than submarines - radar is good at detecting metal objects, but not so good at finding non-metallic objects. Similarly, surface ships using passive sonar or hydrophones would find it difficult to confuse a whale for a submarine. Whales do not make the machinery noise that subs do, and so are difficult to confuse. However, ships using active sonar or ASDIC have been known to confuse echoes from whales and echoes from submarines. As such, during WWII, whales were occasionally depth-charged by surface ships.