Today I took a trip with my wife to Inner Space caverns in Georgetown, Texas. My wife had never been on an underground cave tour and this is the closest one to where we live.
The cavern was found accidentally in 1963 during the construction of US Interstate 35. Here is a photo of the original hole the first explorers climbed down in 1963: 
We went on the 'Adventure Tour' which is the most inexpensive at around 22 dollars and change per adult, and it was a 1 hour guided tour. The entirety of the tour was lit by creatively placed artificial lighting. Though the temperature outside was quite cool, in the low 60s, the temperature remains in the low 70s and very humid at all times throughout the cave system.
Here are a few more photos of the cavern and the incredible stalactite and stalagmite formations which are thousands of years old.
Photos taken with Canon 7D