This is really weird, and you can hardly read about it anywhere, but there's this rather unassuming cave in Dinosaur National Park, just up the track from Echo Park campground. (Which is a small camping paradise, by the way).
Anyway, the entrance to the cave is a fairly small hole at the bottom of a huge vertical wall of rock. Once inside, there's a long arch, and a back wall. But the front wall, arch, and back wall simply aren't touching each other. There's a gap of about a foot between each sheet of rock, and no matter how far you wedge yourself along the cracks, and no matter how bright a flashlight you take with you to look, you can't see an end to the gaps.