This is insanely cool: Discover the Oldest, Weirdest Instrument On Earth: The Lithophone
"Stalactites hang tight to the ceiling, and stalagmites push up with might from the floor: this is a mnemonic device you may once have learned, but chances are you haven’t had much occasion to remember it since. Still, it would surely be called to mind by a visit to Luray Caverns in the American state of Virginia, home of the Great Stalacpipe Organ."




