Kentucky

Mammoth Cave This is the world's longest cave system, with more than 365 miles explored. http://www.nps.gov/maca/ And it is a National Park. The link has plenty of info on the history and tours for those interested. 

Horse Cave, http://cavern.org/hrc/horsecave.php This link gives you much information about the many caves of Kentucky and the vast underground rivers. 

Diamond Caverns has a LOT of great photos and info. Check out the Photo Gallery on the top of the page. http://www.diamondcaverns.com/ Pictures recent and of long ago. New caverns found and protected. 

Lost River Cave and Valley is a beautiful place that boasts the largest entrance to the underground river with 30 minute boat tours through the caverns. http://www.lostrivercave.com/ Click on the History to learn more about the area and river.

Cub Run Cave is near the Mammoth Cave but was discovered in 1950. It was then closed until 2006 and is now opened as a show cave. Lots of pretty lighting but this site has little but tour info. It does explain that after its discovery, it was closed because of a dispute over the surface rights. Visit the Photo Gallery for some good glimpses. http://cubruncave.net/index.htm 

Crystal Onyx Cave is located in Pruitt's Knob, one of several limestone plateaus in Kentucky, which were formed by rivers cutting deep in a huge limestone plateau. The most famous one is nearby Mammoth Cave Plateau. Prewitt's Knob is protected from erosion by a layer of resistant cap rock, and so it stands like an island in the farmland of the sinkhole plain. For a long time, no cave was known in this mountain, but Cleon Turner was convinced there was a cave, so he searched for years. Finally in 1960 he discovered this cave by digging in a promising spot. He blazed an entrance tunnel and opened the cave to the public. Crystal Onyx Cave, as the name suggests, is famous for fine dripstone formations. Onyx is the local slang term for dripstone. The cave has stalagmites and stalagmites, rimstone dams and cave bacon. The most important sight of the cave is a working archaeological excavation. The Indian burial site is dated at 680 B.C.. It is studied by researchers from Washington University. http://www.showcaves.com/english/usa/showcaves/CrystalOnyx.html but there are no photos and little other information

Kentucky State Parks While most all states charge admission to their parks, Kentucky State Parks do not. Reservations are required. 

National Park Directory and look for Kentucky