Russell Cave National Monument is an exceptional park near Bridgeport, Alabama not only for the beautiful landscapes surrounding it, but also for it's interesting history. The cave was used for thousands of years as a shelter for the prehistoric inhabitants of North America and, from a scientific perspective, is one of the most important archaeological sites in the country.
The cave itself is enormous and goes over 7 miles into the earth, making it also one of the largest caves in the country. The interior, though, is delicate so you need to visit with a park guide. You can arrange a visit (they happen several times a day) at the visitors center and explore the park of the cave that was used in prehistoric times and where they've discovered all sorts of artifacts and arrowheads.