The assessment of the stability of underground structures is an interdisciplinary task which consists of geology, hydrogeology, surveying and geotechnical engineering. This thesis is based on the investigation of an abandoned gypsum mine and gives an example of how these disciplines, their methods and the acquisition of data work together to identify weak zones in the underground structure. These data also provide the basis for further analytical and numerical methods. Hereby, the focus lies on the development of a complex three-dimensional numerical simulation model. It demonstrates that a complex layout of old and “grown” underground structures cannot be represented accurately in two-dimensional analyses. Consequently, such models cannot provide realistic and sufficient results for stability investigations of underground structures. Related to the stability of underground structures a non-negligible factor is the influence of water. The occurring leaching process of gypsum leads to a weakening of the surrounding rock which causes a strength reduction. The progression of the leaching process and the export of material are investigated in detail by a new experimental setup. This softening of the material by the leaching process is considered in the simulations. The presented comprehensive approach, including laboratory tests, analytical methods and complex 3D simulations, provides a possibility to predict the time-dependent occurrence of critical zones in the rock mass and thus allows an idea where the support measures should be installed at an early stage.