Sonic booms laid down by Navy fighter aircraft flying at Mach 1.1-1.2 have been observed below the surface of the sea by means of a string of hydrophones 195 feet long dangling from a surface ship. The underwater booms were found to decay about as the -3/2 power of the depth below the surface, to have the same spectral content as the boom in air, and to travel down the string with the velocity of sound in water. These findings contradict the theory of an 'inhomogeneous' wave incident beyond the critical angle, as originally stated by Rayleigh and as recently extended to sonic booms by Sawyers and Cook. They suggest, instead, that the underwater sonic boom is a wave scattered by the rough sea surface into the sea below. Its decay with depth is so rapid that it is not likely to be perceptible against the ambient noise background at depths greater than one or two thousand feet in the deep sea. (Author).