Hollow Moon, Nov20, 1969, Apollo 12 deliberately crashed the Ascent Stage of its Lunar Module onto the Moon's surface; NASA reported that the Moon rang 'like a bell' for almost an hour

Popular Science article from 1970 - https://books.google.ie/books?id=6QAAAAAAMBAJ&pg=PA56&lpg=PA56&dq=popular+science+mar+1970&source=bl&ots=Z3OOgcOG1V&sig=smZ-Uim9lcCPbxHL2IzmvSmvUDQ&hl=en&sa=X&ei=iNR5UPzyHeWViQKSuoDoDQ&redir_esc=y#v=onepage&q=popular%20science%20mar%201970&f=true