Serpentinite fragments were in rock waste from the Nagasawa Ichigo tunnel under construction in the Masuda area, Shimane prefecture, southwest Japan. This finding indicates the existence of a relatively large serpentinite block（or small rock body）within the Jurassic mudstone melange. This finding is important for considering the mantle condition in the Masuda area, because serpentinite blocks of this scale have not been reported previously from the area. This serpentinite has primary geological features of the upper most mantle.
　This serpentinite is distributed from 170m to 480m from the north western entrance of the tunnel. The tunnel rock waste samples represent sections about 10-20m in length. These serpentinites sometimes retain original minerals and structure. Basd on results of preliminary observation under the microscope and analysis of chromian spinels, this serpentinite megablock may originally have mainly consisted of deplete harzburgite（-lerzolite）.