Another 'cool' mega-instrument is the Earth Harp. Instead of transverse waves (plucking) the strings, you use compression waves (pushing) in the strings to make the vibrations. Due to some physics here, you then need a very large resonator to hear the sounds. Essentially the Earth itself. Here's a good video from the Burn at an Earth Harp performance: https://www.youtube.com/watch?v=c66w_pPnO-s