An article in the latest issue of the journal, Studies in Conservation, reports on the condition of the walls of the Great Hall of the Bulls in the prehistoric cave of Lascaux and warns of, "...the appearance of damp areas on the walls due 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 surface in general and, more importantly, the painted figures."*
The threat seems to be from two distinct, and natural, hydrological 'regimes' within the chamber and not to human activity (though human activity outside the chamber might be affecting what goes on within it :-( Whatever, it would be a tragedy beyond belief if these prehistoric paintings were lost.
* Studies in Conservation. Journal of the International Institute for Conservation of Historic and Artistic Works. Vol. 51, No. 4, page 251.