The cave of Lascaux (Dordogne, France) is a reputable place known for its paintings and engravings. Since 1963, its conservation has been given a priority. Many and various sensors have been set into and around the cave to know underground flow conditions, climatic conditions and carbon dioxide fluctuation. For this gas, abnormal concentrations have been recorded in the lower part of the Lascaux cave. The gas is produced by biogenic activity but the source is still not understood. We have been studying the functional relationship between the concentration of CO2 and the atmospheric pressure, by means of the technique of entropy of curves, which leads to a more rigorous statistical analysis. Results show that external parameter such as rainfall could influence CO2 fluctuation and that a sandy-clayey formation, located all around the cave, may be the major source of carbon dioxide.