Louisville Mega Cavern

Next time you are near Louisville, Cincinnati, or Indianapolis, check out the Mega Cavern in Louisville, Kentucky. It's a 100 acre limestone cavern which probably began as a natural cave but was extensively mined for bridge and road construction from the early 1930's to the 70's. Since then, the cavern was rehabilitated, and over 850,000 truck loads of backfill have been dumped to raise the floor to the level that it is at today, to create roads and level warehouse space.

You can bike, zip, and otherwise tour the facility on a tram.

Zzzzzzzzzzzip! The zip tour takes about 2+ hours, extends across maybe 6 or 7 lines and two lengthy foot bridges.

Shit for pic but you get the idea the depths below the zip lines and foot bridges.

The cavern also allegedly houses the worlds longest underground suspension bridge.

"Zipline to Hell" awaits, a 666' zip where you can hover across and holler to your deceased friends members who may be presently burning with the prince of darkness for all eternity.

Beyond recreation, boats, RVs, etc. can be stored without winterizing. They house everything you can imagine storing. The cavern in full capacity could allegedly hold up to 16,000 boats!

The mega cavern is also classified as a building, and is the largest building in the State of Kentucky. Air circulates through by natural air pressure but other mechanisms are in place for mold and that sorta stuff.

Fun fact, officials made plans during the Cuban missile crisis to house 50,000 people in the cavern in case of nuclear attack.

Hope you get to see it one day.
Source: http://www.louisvillemegacavern.com/about/history.html
and many others but I closed my text box and am too damned lazy to find em again....