Apollo debris taught the Moon to ring and revealed its solid, fractured interior

From 1969–1972 NASA intentionally steered spent Apollo hardware into the Moon to serve as calibrated seismic sources; the near-side seismometer network recorded highly diffused, long-lasting signals—nearly an hour in some cases—because the Moon’s dry, fractured megaregolith scatters and slowly absorbs seismic energy, not a hollow shell, producing a bell-like reverberation rather than sharp Earth-like waves. Apollo 12’s LM impact created a ~9-meter crater and the larger third-stage impacts rang even longer; gravity mapping by the GRAIL mission and continued data analysis confirm the Moon is solid, not hollow, though the seismic network logged more than 12,000 moonquakes before going standby in 1977.