The site of the deepest hole on Earth (well sort of, see Wikipedia article for clarification), the Kola Superdeep Borehole on the Kola Penninsula in Russia. Fascinating how completely empty and without fanfare the architecture is.
EDIT: as of January 2014, this is no longer the deepest hole. See this Wired article for info on two deeper holes, and a very nice sound installation created at one of them.
The location of the hole, as seen on Google Maps. Above photo via Wikipedia user Before My Ken.