Abstract: A subsurface seismic source for generating seismic waves comprises a fluid supply conduit section conducting fluid downward and having a seat for receiving a blocking body transported by said fluid. Said body blocks the fluid through said section when forced against said seat by said mud. At least one piston chamber is arranged adjacent to and connected with said section, has in one end wall at one end an opening to said section above said blocking body when forced against said seat, and comprises a biased piston closing said chamber at the other end and being driven to the corresponding other end wall when the blocking body is forced against said seat. Said blocking body is forced through said seat at a predetermined mud pressure level, thereby releasing said piston, the piston colliding against the one end wall and generating seismic waves.