Abstract: An under reamer for expanding a borehole through an earthen formation is taught. The under reamer includes: a mandrel including an end for connection into a drill string and an opposite end and a housing including an inner bore, telescopically disposed over the mandrel and axially moveable relative to the mandrel but rotationally moveable with the mandrel. A plurality of under reamer arms are carried on the housing, the under reamer arms are formed as blocks including cutter-supporting portions in which poly-crystalline diamond compact cutters are mounted. The under reamer arms are moveable by driving the housing axially over the mandrel between a retracted position extending into the bore of the housing and an expanded position wherein the under reamers arms are pivoted out of the housing inner bore and supported therebehind by the mandrel such that the cutter-supporting portions are exposed for use to enlarge a borehole.