Abstract: Process and apparatus for the ultrasonic treatment of underground rocky strata, primarily for drilling therein, and for increasing the permeability of the strata. A flushing medium is made to flow through a mechanical drilling tool such as a rotary drill bit with rock-breaking elements, a pressure wave is superposed on the static pressure of the flowing medium at the location of rock breaking, and the frequency of the pressure wave in the ultrasonic range, preferably between 20 and 100 kHz; good results have been obtained at 27 kHz. The inventive apparatus includes a passageway in the tool for the flushing medium, a resonance chamber in the path of the medium, together with an exciting or interference element, constituting means for changing the flow conditions of at least part of the medium from laminar to turbulent, producing therein a pressure wave, and superposing the latter, as aforesaid. The resonance chamber preferably opens into a jet nozzle forming part of the drilling tool.