Abstract: The present invention provides a pneumatically operated impact-action self-propelled mechanism for driving holes in the earth, comprising a cylindrical housing assembly (1) with an anvil member (2) located at a forward end thereof; and a pneumatically-operated impact piston (3) reciprocal in the housing to deliver successive impacts to the anvil member (2) and forming with the housing a forward chamber (6) of variable volume. The mechanism includes a head chamber (22) forward of the anvil member (2), a head piston (23) reciprocal in the head chamber (22) and connected at its forward end to the head (24) of the mechanism, and compressed air supply member (29, 30) communicating between the forward chamber (6) and the head chamber (22) to the rear of the head piston (23) so as to cause the head piston to travel forwards.