I thought I had been to a good number of the caves in the US, but then I stumbled upon this information: http://en.wikipedia.org/wiki/List_of_caves_in_the_United_States.
And I realized that I had visited just a few - Carlsbad Caverns (NM), Wind Cave (SD), Jewel Cave (SD) Linville Caverns (NC), and now Mammoth Cave (KY).
And I found out that there are over 9500 caves in my home state of TN!!!
I was quite impressed with Mammoth Cave - not just because of the caves, but probably even more so for the above ground features this National Park has to offer. The other caves I have been to have been just that - caves only. But at Mammoth Cave, there are over 75 miles of scenic backcountry trails in the park. We opted to take advantage of those trails and stayed at one of the backcountry campsites, rather than spend the night at one of the three overpopulated and overpriced campgrounds in the park.
Speaking of money, I was completely prepared to pay to enter the park, but it was free. I was completely prepared to pay for the backcountry campsites, but they were free. Like a National Park should be! I've already paid my dues thank you! Of course you had to pay for the personalized cave tours, but the fees were reasonable, and this is the longest cave system in the US - they would have a reason to have astronomical prices if they wanted to.
We also had no problem reserving one of the sites, despite walking up that day to request it, and the park service personnel were so helpful and nice. They are also staying true to the natural habitat of the land. They refuse to build bridges over the rivers throughout the park (although they are in the process of adding on to the really nice visitor's center); instead, you take a ferry across. You drive your car onto the ferry and scurry across. That was neat. Because of the motivation of keeping human impact low, we saw lots of deer and turkey during our weekend stay. Speaking of rivers, canoe and kayaking options abound as well, and you can couple a multi-day river run with camping options throughout the park, with very little restrictions.
All in all, I plan to head back to Mammoth Cave, not so much for the caves as for a long weekend backpacking trip.