#2 Lascaux Caves, France
Located in Southwestern France, the Lascaux Caves contain 900 of the best preserved examples of prehistoric art. The caves have been banned to the public since 1963 as they have been menaced by a series of unexplained fungal invasions. Any human presence in the caves is regarded as potentially destructive. Normally, the caves are entered only once a week by a security guard for a few minutes at a time.