Yeah, sitting on top of the Earth's crust can be fun, but ducking a bit below can be adventure. Recently the 2013 "Inside the Volcano" tours were announced by Iceland Naturally:
For the second year, visitors to Iceland will have the opportunity to explore the inside of a volcano with the appropriately named Inside the Volcano tour. The tour takes visitors to the dormant Thrihnukagigur Volcano, located approximately 20 miles from Reykjavik, Iceland's capital (see a map of Iceland).
You don't have to get inside a volcano to get your kicks--one of the most interesting natural wonders in Italy is the Grotte di Frassasi, a huge cavern in the Marche region.
There are also crypts of churches, which usually contain remnants of earlier churches or pagan temples.
Get more ideas for launching your underground vacation: Beat the Summer Heat on Your Vacation!