This book includes own research studies and results regarding, on the one hand, to the problem of stability - reliability of mining works in difficult conditions in geomechanical terms, and on the other hand, the research had also been subject to exploitation of rock salt, meaning micro and macroscopic behavior analysis of salt based on detailed laboratory geomechanical research and in situ, but also by analyzing the stability of underground workings executed in salt massif during operations, and the idea that underground voids created will have another destination after the exploitation of salt deposits. Particularly, the results of observations and in situ measurements performed in underground are presented, the correlation between the parameters that influence the stability of underground works, critical depth from which one can define the vertical behavior of the rock deformation, that is, limits the separation zone from the elastic behavior of the plastic behavior. A mathematical model was developed, which is based on the concept of entropy, a model that can analyze the stability of the pillars of any shape.