For eight years the Moon had a working seismic network. It was switched off in 1977 to save about a million dollars a year.

Apollo 12, 14, 15 and 16 left seismometers that ran continuously and recorded thousands of events. Funding ended and the stations were commanded off. The tapes are still being reprocessed. Most of what we know about the lunar interior comes from that eight-year window and nothing has replaced it since.