What is the most famous subterranean place?

You might want to define this as being natural or artificial. I'm leaning towards subway lines. Especially if you look at the number of "visitors" per year. The oldest subway is in London. The most beautiful subway is in Moscow. A good, well used, but more plain subway would be the New York Subway. Other places of note: Valley of the Kings near Luxor Egypt & the Tomb of King Tut. The Indian Jones Film made a lasting impression on the world with the Canyon of the Crescent Moon, Petra, Jordan. I'm trying to think of a few caves. Most of them are relatively local and may not have particular national or global interest. The Sea Lion Caves in Oregon is quite interesting.