drill string 26 and the fluid return the control unit for the units 10 may comprise a wire line recoverable probe which is pumped down the center of the drill string 26 and having pre set battery powered controls for regulating the magnitude and direction of the thrust supplied by the units 10 as well as the timing sequence of its pulsations existing wire line telemetry systems could be utilized or remotely operated two way wireless telemetry along the drill string could be employed the control logic may also be solid state electrical fluid logic or rotary valve an instrumentation survey and telemetry package could be located at 34 between non magnetic drill collars 36 the control and earth penetration units on either side of the muscle units 10 according to the present invention can take a wide variety of forms as exemplified by the structures of fig2 b 2d the device 39 of fig2 b is especially adapted for horizontal drilling for instance into a coal seam a drag bit 40 powered by a rotating drill string 42 which is operated from the surface may provide the earth penetration for the device a suitable worm power package could be located at 44 and a suitable instrumentation package 45 could be located at 46 the instrumentation package 45 is located in a plenum chamber in section 46 of the device non magnetic drill collars 47 are provided on either side of the instrumentation package 45 in order that the drill string does not interfere with.