Shadows flickered on the walls of the narrow cavern as we groped and felt our way forward, following our guide's lantern deeper into the Grotte de Font-de-Gaume. Our eyes were still not accustomed to the low level of light and the going was slow. Fifty meters into the cave we encountered the narrow Rubicon, a meter-wide passageway where we squeezed single-file between limestone walls to reach the main gallery.