Ask HN: Underground Vacation Destinations?

During 2007-8 I spent most of my sophomore year at Rutgers University in what students called "the catacombs" on Livingston campus: a large network of underground tunnels connecting the dormitories to one another and featuring underground study rooms, recreation rooms, etc.

Ever since then I've found myself drawn to structures like this. Does anyone know of a vacation destination like this?

I know this sounds crazy, but I'm not really looking for caves or natural underground structures.