To minimize influences on the environment and reproduce the environment where benthic organism are easy to live.

This system is capable of feeding high-pressure air from the undersurface of a casing 21 simultaneously with drilling of a continuous auger 23 with an air hose 36, jetting high-pressure seawater from vertical nozzles 29 and an oblique nozzle 30 under the hollow shaft 24 of the continuous auger 23 and thereby stirring a silt layer together with the drilling of the silt layer with the high-pressure seawater. The activity of microorganisms is stimulated by mixing and replacing of both the layers and the living environment for shells, lobsters, etc., which are benthic organisms is improved.