 uh... uh... there were seismic seismometers that were placed on the moon during the apollo missions multiple ones so they could really good data and then they crashed saturn five upper stages into the moon to make those uh... uh... seismometers get data and from that data uh... uh... uh... uh... uh... researchers who study seismology were able to get a very clear idea of the interior of the moon and it's generally solid