For subtropolis, see http://en.wikipedia.org/wiki/SubTropolis. From there, http://www.huntmidwest.com/subtropolis/index.html gives you the site of the business. I find their brochure gives a good idea about what it looks like: http://huntmidwest.com/pdfs/subtropolis_brochure_2012.pdf

Impressive, but way more industrial than, for example, the centuries old limestone caves in the Belgian-Dutch-German border area (http://maastricht-students.com/2012/07/millions-of-years-of-..., http://www.tripadvisor.com/Attraction_Review-g188575-d318667...)