One of the reasons Isaac and I went to Cuyahoga Valley National Park yesterday was to obtain an America the Beautiful Access Pass. The National Park Service provides these passes to people with permanent disabilities. The pass is good for a lifetime and it allows free admission to all national parks and discounts on many activities such as camping, boating, cave tours at Mammoth Cave, etc. and many (but not all) national parks and national forests.
The pass is free if you get it in person at any national park office. You can also request one by mail (see the National Park Service website for an application) for a fee of $10. You need a photo ID, like a driver's license or state issued ID card, and proof of your disability, like an award letter from Social Security or the VA or a letter from your doctor. The National Park Service website lists all the forms of verification they accept, as well as a few things they don't accept (like a handicap parking placard).
We are planning a trip to Mammoth Cave this summer and with the access pass, will get 50% off on campground fees and cave tours, as well as free entrance to the park. And yes, they do allow service dogs on cave tours. I am curious to see what Isaac thinks of the cave!