At the base of a cliff in the Ardeche region in southeastern France last December, the three middle-aged spelunkers felt a breeze wafting from a pile of rock and debris. "That was a sign that there was a cave beneath it," recalls Jean-Marie Chauvet. With his companions, Chauvet cleared away an opening, then wriggled through a tunnel into a complex of large caves.
Then, in the pale glow of their head lamps, the explorers noticed two red lines on a cavern wall. Chauvet, a government employee who oversees the protection of the many historically important caves in the region, recognized the...