the mountains and the canyons started to tremble and shake
On 9 February 1971 the Sylmar earthquake hit the San Fernando Valley in the early morning, and later that day Jimmy Page flew into Los Angeles with engineer Andy Johns to mix an album that already contained a song about California shaking itself apart. Aftershocks were still arriving. Page listened to what he had on tape, heard Robert Plant sing about the mountains and the canyons starting to tremble, and quietly decided to mix that one last — no point, as he saw it, in tempting the ground twice. Johns told him not to be so bloody stupid.