Sal Hollow is a singletrack hike/bike trail located in Mammoth Cave National Park--one of very few mountain bike trails inside a national park. This is out-and-back route (both ways it's about 16.6 miles) is mostly flat and smooth, but occasional log crossings and rock gardens keep you on your toes. Experienced riders will enjoy picking through the obstacles, and beginners looking to test their skills will meet some good but often doable challenges. (If not, it's easy to bail, and there's plenty of smooth singletrack for everyone between.) This trail is the only non-horseback trail in the park and is always in great shape, even after a rain. Be sure to stop by the "cave" entrance near the half-way point or take a park cave tour after the ride. Apparently, the aptly named Mammoth Cave is the world's largest known cave. Echo!