Mammoth Cave National Park is famous for its below ground wonders, and that's a little unfair, because the quality of its above ground backcountry, while not widely know, is pretty good for this part of the Midwest. If you'd rather share your hiking with mountain bikes rather that horses, then this is your trail. In fact, until the Park Service opens its new trails, this is your only "no horses" trail in the Park. Only the last mile of the trail allows equestrians, who pretty much rein over the rest of the park.
Like many trails nearby, the Sal Hollow trail begins at the Maple Springs Trailhead. Unlike these other trails, this one does not follow a roadbed for any of its 8.1 miles. I understand that this trail was designed for mountain bikes, so you'll find a well marked, easily followed, single track path for its entire length. The trail is goes up and down through a forested Karst region crossing several stream beds for its first 3.5 miles. During my trip, three of these streams were wet and all were easy rock hops. After crossing the Turnhole Bend Trail at 3.5 miles the trail gets a little more elevation change in places, but remains a moderate hike. There are less wet stream crossings on this half of the trail, but one particular crossing features a rock shelf with a cave visible from the trail complete with water rushing out. Look for this feature to the north of a prominent stream crossing at about mile 5. Fill up your water bottles here. If you're continuing on, it's your last chance to tank up for a while. Although it's not a very attractive, the Sal Hollow Campsite at mile 7 or so would make good place to stop for lunch or even to spend the night. It's up a hill off of a spur trail. A permit is required for overnight camping.
You can turn around here, or better still, continue on to the Buffalo Trail and follow it back to the trailhead to make a loop. It's about 11 miles round trip this way. Other options are possible.