The Lascaux caves are located in France, near the village of Montignac in the Vezere Valley. They became famous when numerous Paleolithic cave paintings were discovered adorning their walls. These works of art, realistic portrayals of a variety of large animals, are estimated to be as much as 20,000 years old. The site, together with a number of other caves, has been designated a UNESCO World Heritage Site and is known as Prehistoric Sites and Decorated Caves of the Vezere Valley.
The paintings had been preserved for all this time, protected from the outside by the natural sealing of the caves. Once discovered in 1940, though, they began to deteriorate as a result of the moisture, carbon dioxide, and other environmental changes resulting from air circulation and numerous visitors to the caves. So, a replica has been constructed for visitors, and only qualified researchers are given limited access to the actual site.
Seems like a good plan, since it allows the public the chance to view replicas with lots of historically accurate context, not just pictures on the wall of a museum or some "artist's impression" of what the original site looked like. And hopefully they will succeed in preserving the originals, which offer a tangible link to the very early history of humankind.