Subterranean quarries were the source of Paris buildings. Today those places are a touristical attraction. The subterranean burrow system amounts more than 300 kilometres.
Les Catacombes also served as gravesite until the 18th century, because Parisian cemeteries were quite overcrowded in those times. The bones of former generations can be found decorative stacked in the areas which are opened for the tourists.
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Metro 4 and 6: Denfert Rochereau