On a recent trip to Texas, I stopped by the Inner Space Cavern near Georgetown for a guided tour of the cave, and spent a few hours admiring all the limestone formations (and fossils!)
The cavern is thought to be about 100 million years old, though the vast majority of that time has been spent sealed off from the world, deep underground.
Between 45 and 20 million years ago, during the Ice Age, sinkholes opened up which provided an entrance to the outside world - evidenced by the remains of the unfortunate mammoths, saber-tooth cats, and peccary who fell in (presumably trying to drink from water-filled sinkholes) and were unable to escape.
About 14,000 years ago all the natural entrances closed up again, and the cave was again lost to time -- until the 1960s, when a Texas Highway Department crew accidentally drilled through the ceiling and lost an expensive drill bit, promptly sending one of their men down to retrieve it. Thus the caves were discovered.
And here is the tale of the discovery, written by the second person to ever enter the caves, Texas Highway Dept geologist James Samson. I shall leave him to tell the story, since I cannot hope to match it!