The Moon Rings Like a Bell

When Apollo astronauts left seismometers on the Moon and later sent spacecraft to crash into its surface, they detected vibrations lasting for an hour or more. Unlike Earth, where seismic waves fade quickly, the Moon’s cold, dry rock lets vibrations keep bouncing around for a long time, so it seemed to ring like a bell. Scientists use those recordings to probe the Moon’s interior, which has a liquid outer core—so it isn’t hollow, despite a claim often misused in conspiracy theories.