It's in Big Cottonwood Canyon and features a rush of water falling through a pothole bored into the top of the cave. The result is a beautiful, subterranean pool inside the cavity. To get to the trailhead drive nine miles up Big Cottonwood Canyon to the Mill D Trailhead. Take a right towards the Jordan Pines picnic area. Follow the signs to Donut Falls. The hike itself is about 3.5 miles round trip.