This F/A-18 Hornet was photographed at the moment it broke the sound barrier. Many people have heard a sonic boom, but few have seen one. When an airplane travels at a speed faster than sound, sound waves produced by the plane cannot precede the plane, and so accumulate in a cone behind the plane. When this shock wave passes, a listener hears all at once the sound emitted over a longer period: a sonic boom. As a plane accelerates to just break the sound barrier, however, a mysterious cloud sometimes appears. So far nobody has been able to explain this remarkable phenomenon.
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Pic credit: Ensign John Gay, USS Constellation, US Navy