The Sub‐bottom Depth Recorder (SDR) has been developed to study the underlying geologic structure of water covered areas. The SDR is essentially a broad‐band, high‐powered echo sounder which utilizes one of two sound sources. The first, an electrical spark discharge (Sparker), has produced penetrations in excess of 600 ft in 50 ft of water. The second, a combustion chamber using a mixture of propane and oxygen (RASS), has produced penetrations in excess of 1,400 ft in 80 ft of water. Results obtained from the SDR have been found to be in excellent agreement with test boring and seismic refraction data.