An apparatus (10) is provided for detecting deformation in solids in deep holes having a resilient vessel (12) pre-stressed into a rock formation. The vessel (12) is filled with fluid (14) and is sealed except for an outlet tube (16) of small diameter which extends all the way to, or close to, the surface (18). There the outlet tube (16) terminates in an extensible bellows (20). The whole apparatus is completely filled with fluid (14). Any deformation of the vessel (12) will cause fluid to be forced through the outlet tube (16) and so cause the bellows (20) to extend or contract. This motion can be monitored by any position-sensitive transducer (22) such as a linear variable differential transformer or capacitance bridge. A second tube (24) of the same length and size as the outlet tube (16) is filled with the same fluid. This tube (24) terminates at the bottom in close proximity to the resilient vessel (12). It also has a bellows (26) and a displacement transducer (28) at its upper end at the surface (18).