This is Cobb Cave in Lost Valley National Park near Ponca, Arkansas, a popular launching spot for the Buffalo National River. And it is ENORMOUS. In fact, it wasn't until I got home with my images and began stitching this panoramic together that I realized just how big it really is.
To really give a sense of its size, there are 5 hikers in this photograph. Can you find them?
I'll even give you a head start by saying that the person on the left is 
Here's a link to the full-size image to help in your search. :)