Abandoned underground mines appear to be very sensitive to the variations in their environment. Due to seasonal environmental changes often related to climate and resulting in changes in the water table and hygrometry, the pillars are often subjected to cyclic variations of saturation. This could affect the short- and long-term stability of these abandoned structures. In this paper, a preliminary study on the behaviour of Château-Landon chalk has been carried out. Samples were extracted from pillars of the abandoned Royer chalk mine (Château-Landon, France). The mechanical behaviour of this chalk was then investigated by performing conventional hydrostatic, triaxial compression, and creep tests under drained conditions at different water saturation degrees and confining pressures. The obtained results show up fundamental aspects of this chalk behaviour. Its high sensitivity to water and confining pressure are described, and a water saturation-induced plastic deformation is analysed. The results presented in this paper provide some key mechanisms that can be used in the framework of back analysis and expertise in underground chalk mines, as well as the data required for the development of a specific constitutive model.