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!