Abstract: A deep-drilling system has a vertical tubular drill string having an upper section and a lower section, a rotatable bit at a lower end of the lower section so that when the string and bit are lowered into the ground longitudinally they form therein a bore. A drill mud is pumped at a working pressure longitudinally through the string to emerge at the lower end of the string and flow upward in the bore around the string. A hydrostatic compensator joint in the string between the sections is centered on an upright axis and has a tubular upper part, a threaded joint rigidly connecting the upper part to a lower end of the upper section, a tubular lower part, and a threaded joint rigidly connecting the lower part to an upper end of the lower part. Each part has surfaces directed radially and axially at generally complementary surfaces of the other part. The axially directed surfaces of the upper part are spaced axially from the respective surfaces of the lower part by at most 5 mm.