Why is it difficult to study the formation of the earth and its structure?

The earth's surface is predominantly made up of rocks which are thick in diameter and difficult to break through. During the earth's formation, these rocks were liquid in form, due to excessive heat. Over time, the temperatures cooled down, allowing the rocks to solidify and heavier rocks to sink down to the center of the earth. The lighter rocks remained on the surface and broke down to create soil. Scientists have attempted to dig further into the earth to discover the deeper layers, lying beneath the crust, but they are incapable of digging deep enough. This is due to the solid rocks, the thick diameter of the crust and the excessive heat and temperature in the deeper layers which are closer to the core. To study the formation and structure of the earth, scientists need to rely on earthquakes which create cracks in the earth's crust and study samples of magma.