Abstract: A pneumatically operated impact-action self-propelled mechanism for driving holes in the earth is disclosed. The mechanism comprises a cylindrical housing with an anvil member located at the forward end thereof. An impact piston is reciprocal in the housing to deliver successive impacts to the anvil member and shapes with the housing a forward chamber of variable volume. A control assembly comprises a forwardly extending sleeve which is slidably received within a rear space of the impact piston to form a rear chamber of variable volume. A central passage is connected to the sleeve for continuous supply of compressed air into the rear chamber and therefrom into the forward chamber through apertures in a side wall of the rear chamber of the impact piston. An arrangement is provided for lockably locating the sleeve longitudinally with respect to the apertures for providing forward or reverse movement of the mechanism.