A system for laying a subterranean water impervious asphalt layer which is mounted on a tractor and includes a liquidized asphalt supply tank and a subterranean plough provided with a plurality of outlet ports so that liquidized asphalt is fed from the tank through a liquidized ashalt supply pipe connecting the tank to the ports by a liquidized asphalt supply pump provided in the pipe and driven by a hydraulic motor so that liquidized asphalt is discharged by the ports to form a water impervious asphalt layer into a subterranean cavity. The system is provided with a hydraulic pump driven by an engine of the tractor and connected to the hydraulic motor and a switching valve provided in the lines connecting the hydraulic pump to the hydraulic motor so that the hydraulic motor is driven in the forward or reverse direction as well as is held standstill by the actuation of the switching valve. The system is provided with a washing tank containing cleaning liquid therein, and the tank is switchingly connected to the tank through a two-way switching valve so that the pipe and the ports can be cleaned by the cleaning liquid by operating the two-way switching valve. The system further comprises a burner for heating asphalt in the tank which is supplied with fuel from the fuel tank of the engine through a fuel supply pipe by the actuation of a cock provided in the fuel supply pipe.