On a chilly autumn day in September 1940, 18-year-old Marcel Ravidat was walking through the dense forests of Montignac, France, with his dog Robot. When Robot began sniffing around the hole left by an uprooted tree, Marcel discovered a dark passageway descending 50 feet into the ground. Marcel believed he had found the secret passage to the legendary Lascaux estate, but what he had discovered was much more important.