Abstract: A rod extensometer which can be inserted into a hole drilled into the roof or floor of an underground excavation to monitor the shifting of material surrounding the excavation. The extensometer has a head which is anchored in the inner end portion of the hole by a hardened synthetic plastic substance, an annular section which is inserted into the outer end portion of the hole and is bonded to surrounding material, and a set of interconnected coaxial cylindrical rods one of which is rigid with the head and another of which extends through the annular section. Changes in distance between the outer end of the other rod and the annular section are indicative of movement of material surrounding the inner end portion of the hole relative to material surrounding the outer end portion of the hole and/or vice versa.