TOBI is an instrumented vehicle which is towed close to the bottom of the deep ocean from a ship, and uses sound to form detailed images of the sea floor. The paper describes the mechanical design of this vehicle and its instrument package. The way in which sonar images are formed is discussed and the acoustic design parameters are considered. Signals are frequency multiplexed on to 10 km of armoured coaxial towing cable, which also provides the power to the vehicle and a two-way communication link. At the ship these signals are separated, displayed and logged onto magneto-optical cartridges. Examples of results are shown and some future developments are identified.