How do you get in an underground city?
There are many underground cities in the world, some are here in the United States. Some are from ancient times and others are modern. Basically, an underground city is a subterranean chamber that may contain living spaces, work spaces, even shopping malls. An underground city can also refer to a transit system like subways. They can also be drainage channels or a network of tunnels connecting buildings beneath street level. They can usually be accessed through the public buildings above them and may have separate entries also. Some underground spaces are very old and require special permission to enter.
You may view an article on underground cities on Wikipedia here: http://en.wikipedia.org/wiki/Underground_city. This article contains a long list of underground cities throughout the world.