Abstract: A apparatus is provided for detecting deformation in solids in deep holes having a resilient vessel pre-stressed into a rock formation. The vessel is filled with fluid and is sealed except for an outlet tube of small diameter which extends all the way to, or close to, the surface. There the outlet tube terminates in an extensible bellows isolated from the atmosphere in an enclosed sealed vessel. The whole apparatus is completely filled with fluid. Any deformation of the vessel will cause fluid to be forced through the outlet tube and so cause the bellows to extend or contract. This motion can be monitored by any position-sensitive transducer such as a linear variable differential transformer or capacitance bridge. A second tube of the same length and size as the outlet tube is filled with the same fluid. This tube terminates at the bottom in close proximity to the resilient vessel. It also has a bellows and a displacement transducer at its upper end at the surface housed in an enclosed sealed vessel.