In addition to the many subway systems in large cities around the world, there are also man-made underground cities, such as Crystal City, Virginia and Underground Atlanta that are hubs of retail commerce.
|Crystal City, Virginia|
Though tourists don't go down there, Disneyworld has it's own underground city beneath the Magic Kingdom that employees can walk through to get to the various sections of the park.
|The Underground Kingdom in Disney World|
The History channel used to have a show called Cities of the Underworld that highlighted underground cities and structures of historical signifcance, such as the Edinburgh Vaults in Scotland.
In addition to retail cities and historic places, there are also massive underground storage facilities such as the Springfield Underground in Missouri.
But mother nature was creating underground worlds long before man ever did.
Aktun Chen is a beautiful undergound system of caves and rivers (called cenotes) in the Yucatan peninsula of Mexico.
As beautiful as Aktun Chen is, Mexico has an (arguably) even more beautiful underground spot -- the Marietas Islands -- which basically has an underground paradise.
Video of the Marietas Islands.
Son Doong Cave
The largest known cave in the world is the Son Doong cave in Vietnam. Although locals had known of it's existence for a while, outsiders only discovered it in recent years. It had remained undetected so long because it is in a remote jungle and is not visible from the air and you have to be close to it on the ground to see it.
|Son Doong Cave|
The cave has trees and the humidity rises into cool air and forms clouds inside.
It may be about as close as you are going to get to Jules Verne's fantasy world in A Journey to the Center of the Earth....unless they find something even bigger.
Despite being as massive as it is, Andy Eavis, president of the International Union of Speleology, says bigger caves are "amost certainly" awaiting discovery and said satellite imagery hints that larger caves may be deep in the Amazon rainforest. See more photos at National Geographic.