The hollow moon theory is a tough one to swallow. I know about the unusually low density of our moon. I know about it's super tough surface rich in chromium, nickel, iron, titanium & silicon. Our moon is a tidal locked moon, so what? The only real oddity about our moon is it is the only symmetrically round moon in our solar system. As far as the "moon ringing like a bell" when the lift off vehicle(s) were crashed onto the surface of our moon that could(?) maybe be explained by a lack of water on/in our moon itself(no water no hydro absorption of impact wave energy). We have conformation of lava fields on our moon & lava fields on a planet/rock takes a HOT CORE which makes the hollow moon theory DOA!!!