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Observations of the condition of the walls of the Salle des Taureaux (Great Hall of the Bulls) in the prehistoric cave of Lascaux showed important moisture variations over time. These are related to the presence and activity of four discontinuities, or joints, within the Upper Cretaceous limestone. There are two distinct hydrogeological regimes in the chamber: one corresponds to a superficial water table with a significant discharge; the other is caused by the activity of the joints, which cannot be measured. Observations of the activity of two of the joints (nos 3 and 4)for the period 2000-2004 showed that the activity of the joints is not always in phase with that of the water table and that it cannot be directly linked to the rise of the water table. These two joints are responsible for the appearance of damp areas on the walls that are subjected to the microclimate in the chamber. If water evaporates from the walls, this could lead to crystal growth and the appearance of a calcite film obliterating the rock suite in general and, most importantly, the painted figures. [ABSTRACT FROM AUTHOR]