In the fall of 1940, a group of boys exploring the outdoors in the Dordogne area of southwest France came upon the entrance to a cave–and unwittingly discovered a treasure trove of prehistoric art. The walls of the cave now known as ‘Lascaux’ are covered with hundreds of images–giant drawings of bulls, horses, and humans–created some 17,000 years ago, in the Upper Paleolithic Period. The cave was opened to the public in 1948, but after several years, scientists observed that the artwork was being damaged by carbon dioxide, heat, humidity, and other contaminants produced by an average of 1,200 visitors who explored the caves each day. To protect the prehistoric masterpieces, the cave has been closed to the public since 1963. Today, the closest you can get is viewing full-scale replicas at the International Centre for Cave Art in nearby Montignac, where our homepage image was photographed.