Seismographic instruments placed on the Moon by the Apollo astronauts determined that the satellite is mostly solid, with a crust, mantle, and core.
Scientists are not sure what the core is made of, or whether it is at least partially molten, or melted.
We do know, however, that it is made of the Moon’s densest elements and is about 800 miles (1,400 km) in diameter.
Solid rock makes up the mantle, some 600 miles (1,000 km) deep.
The hard, rocky crust is about 40 miles (60 km) thick on the side that faces Earth, and is thicker on the far side.