Sand Cave in Cumberland Gap National Park, and a friend standing in the perfect spot. Photographer's heaven.
UPDATE: Wow. I leave my computer for a day and I'm on the front page of Explore. Sincere thanks for all the comments and faves. I've now geotagged the photo if you want to see where I took it.
UPDATE 2: Some people really want to visit this place and have asked for more specific hiking instructions, so here you go:
It's in Cumberland Gap National Park, right at the conjunction of Kentucky, Tennessee, and Virginia. The cave is on the North side of a ridge. The ridge itself is the state line between Kentucky and Virginia. The cave is on the Kentucky side. If you go to Ewing, Virginia, there's a trailhead at a place called "Civic Park".
You hike up to the ridge and follow the signs at the top, which take you back down to the cave. It's a nice day hike, though we spent the weekend on the ridge overlooking the valley, at a place called White Rocks. The downside is that the clouds that made the photo a really cool shot were there all weekend, and we only saw the view of the valley from our campsite for a few minutes on the last morning.