The first hints that the moon had an Earth-like interior came from NASA's Apollo missions. Data gathered by the lunar landers' instruments suggested that the celestial body was differentiated — or layered with denser material at the center and less dense material nearer the surface — as opposed to uniform rock all the way through. Apollo astronauts even left seismometers on the moon, which later revealed that it experiences moonquakes... However, scientists were only recently able to sort through the massive data sets from the Apollo missions and other lunar probes to get a clearer picture of the moon's insides....