Final answer:
The Earth's internal structure includes the crust, mantle, and the inner and outer cores, studied through the measurement of seismic waves, which indicate solid and molten layers.
Step-by-step explanation:
The internal structure of the Earth is complex and consists of several layers: the crust, the mantle, and the inner and outer cores. The crust is the thin outer layer where we live and is divided into continental and oceanic types. Below the crust lies the mantle, which makes up a very large portion of the Earth's volume and behaves in a malleable plastic manner despite being solid.
These layers have been studied through seismic studies, which measure the transmission of seismic waves generated by earthquakes or explosions. As these waves travel through different layers, they provide clues to the composition and state (solid or liquid) of each layer.