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Yeah, it was a couple of old Apollo modules I think they crashed into it, to measure the seismic activity and get more info on the core. The moon did reverberate a bit more than they expected, but it all had logical scientific explanations.

Doesn't make as good a story as saying the moon was stolen by ETs and replaced with a hollow spy probe lolol

"logical scientific explanations"

aka "OH, shit guys! We better come up with something that sounds plausible to explain this!"

ha ha, the earth has a lot of water in it which naturally damps vibrations. The moon is bone dry and rings like a bell when struck, it's solid rock