Although we cannot dig holes deep enough to reach the mantle layer, it is not difficult to prove that the Earth has layers by other techniques. For example, we have been using earthquake (or seismic) data for many, many years to calculate what kind of material is under the Earth's surface. The earthquake data has told us that there is a solid (rocky) mantle layer beneath the crust of the Earth. Under the mantle, there is a layer of liquid metal called the outer core. Finally, under the outer core, there is a solid inner core made out of metals like iron. Seismic (earthquake) information allows us to prove that these layers exist because the boundaries between layers have very sharp contrasts--meaning we actually "see" the boundaries between these layers after we analyze the earthquake data.

Another reason why we know the Earth has layers is because the Earth has a magnetic field. The only way to have a magnetic field around the Earth is to have convecting (moving) liquid metal inside the Earth. This proves that the outer core (which is made of liquid metal) exists. If the Earth did not have a magnetic field, the harmful energy from the Sun would reach the Earth's surface and destroy most living things--including humans. It's a good thing we have a liquid outer core so that we can have a magnetic field to protect us!

One thing that can make earth science difficult is that we are limited by what we can see. It's easy to study the crust because we are standing on it and can measure it, but we can't see deep inside the earth so we have to find ways to measure it without seeing and touching it. But sometimes we can see the layers even at the earth's surface. Mountains are sediments from under our feet that have been thrust above the surface and they show us the crustal layers. All mountains show layered rocks and we know that this is what the earth looks like below our feet (we just can't see it).

It's not hard; in fact it's easy. Just look outside at most mountains, and you can see the layers.