I recently went on a tour of Project Riese and it got me wondering about whether there are actual active underground cities I could visit. Is there a place that's available not only for a tour, but where one could live for some time without coming back to the earth surface?
Follow-up question: Where can I find a map of the Underground City (Beijing)?
You might be interested in Coober Pedy, South Australia.
Coober Pedy is renowned for its below-ground dwellings, called "dugouts", which are built in this fashion due to the scorching daytime heat.