Abstract: A liquid leakage detection system for use in underground storage tanks includes at least two vertically-disposed tubular members mounted to be partially immersed in liquid in the tank. A valve selectively opens and closes the lower end of at least one of the tubular members and air-bleed valves are connected to the upper ends of at least two other of the tubular members. With the air-bleed valves closed, a differential pressure transducer is connected to sense the gas pressure in the tubular members. A control system selectively operates the valves so that the pressure transducer indicates changes in liquid levels within the tank caused by leakage.