Paris is built upon former Parises. Every once in awhile a giant sinkhole opens up unexpectantly to reveal a previos version. I can't remember if this aided in

Paris is built upon former Parises. Every once in awhile a giant sinkhole opens up unexpectantly to reveal a previos version. I can't remember if this aided in the construction of the Métro or Paris's mail system (ruuuubes! A series of them!) but it might have.