This is a great question! Although humans only interact with the surface portion of the earth and thus cannot see the inside of the planet, we know that the earth has a complex internal structure.

The Earth formed through the accretion of early solar system material, and the violent collisions associated with this process led to the planet being extremely hot and molten early in its history (~4.5 billion years ago!). Combined with the force of gravity, this allowed dense material to sink to the core of the earth and less dense material to rise towards the surface. This process is called differentiation, and it happens to other planets in addition to Earth.

The process of differentiation (essentially, the separation of the planet into layers) has created the internal structure of the earth that scientists now detect using seismic waves. If you were to travel through the earth to the other side, the first layer to travel through would be the continental crust (assuming you start on land and not in the ocean, which has a different, thinner sort of crust). On average, continental crust is about 35 km thick, and it is less dense than the other layers.

At the base of the crust, you would next reach a region of the earth called the mantle. This layer is about 2,900 km thick and it makes up most of the planet by volume. The mantle is more dense than the crust, and it would initially be rigid close to the crust (this part is called the lithospheric mantle, which means it is rock-like).

Moving deeper into the earth, the mantle would become less brittle. At the base of the mantle, you would reach the earth's core, which is about 3,500 km thick. The core is mostly made of iron and nickel, so it is very dense -- this is why it sank to the center of the earth early in its planetary history. The core is much hotter than the mantle, and its outer layer is liquid. At the center of the earth, there is a solid inner core. This would be the final layer you would reach before moving through the layers again in the opposite order.

There are other compositions and structures you might encounter (for example, large slabs of oceanic crust that are subducted back into the mantle, or hot and chemically distinct 'blobs' of material at the base of the mantle). These sorts of structures are studied by geoscientists in order to better understand what the planet is made out of, and how the planet has changed over time. The pressures and temperatures in the earth's inner layers are much, much higher than humans could ever sustain -- this is why scientists must instead study the center of the earth using seismic waves or by measuring the chemistry of rocks that originated from the interior of the earth. However, most geoscientists would very much like to travel through the Earth to see its structure up close if it were possible!

I wish I could go to the center of the earth!

On a journey to the center we would first zip through the earth's crust. We live on this layer, but we haven't drilled through it, even though it's the thinnest one!

After the crust we would drill through the earth's mantle. Diamonds form in the earth's mantle, so you could look for some. But you might find it difficult to look around at first because everything would be dark. Deeper down in the mantle, however, you would actually see the rocks glow from the heat.

Bellow the mantle we would travel into the earth's outer and inner core, where the heat and pressure cause strange things to happen. The temperature keeps the compounds in the outer core from solidifying. Flowing material of the core generates a magnetic field that protects us from solar wind and radiation. In the inner core the higher pressure causes everything to be solid. But don't let that fool you into thinking the inner core is not that hot. In earth's inner core the temperature hovers near a scalding 10,000 degrees Fahrenheit!

More advanced students interested in this question could go here: interior of the earth

A: Earth has four layers:
1. a rocky outer crust,
2. a hot and plastic, but solid, rocky mantle
3. a liquid metal outer core
4. a solid metal inner core

Temperature at the center is about 8,000 degrees Celsius. The only reason why the inner core and even the mantle are solid is because of the tremendous pressure. Putting mantle rocks under less pressure will cause them to melt, and that is where molten magma comes from.

If you could hypothetically dig your way through the center of the earth and out the other side, you would pass through 4 main layers . You would start on the crust, which is what we're standing on right now. The crust is made of rock that is relatively cold and can be described as "brittle" which means that it's hard and doesn't bend or flow easily. Think of putting a piece of gum in the freezer. When you take it out, it would be so cold that when you try to bend it, it would snap in half. That's what "brittle" means and that's the reason we have earthquakes - when you put stress on parts of the crust it has the tendency to snap, creating an earthquake.

After digging straight down for about 20 miles you would arrive at the mantle. The mantle is also made of rock (not magma, like many people believe), except here the rock is very hot so it can bend and flow. We call this "ductile" and it's like what gum does when you stretch it at room temperature. Mantle rock is called peridotite and it is mostly made up of the mineral called olivine. Olivine is the birthstone for the month of August (sometimes called peridot). So the mantle is really green!

At about 1800 miles down in the earth you would finally hit the liquid outer core. The outer core is made up of molten iron and nickel metal. We know that it's liquid because some types of seismic waves cannot travel through liquid. When earthquakes happen in the crust, the waves travel through the center of the earth and out the other side, so when we measure these waves we see that some types of waves never make it to the other side because of the liquid outer core.

Finally, after swimming through the liquid metal, you would reach the inner core. The inner core is made up of the same stuff as the outer core - iron and nickel - but the inner core is solid instead of liquid. This is because once you get to the really high pressures of the inner core, the iron crystallizes into a solid. You would have to dig for about 4000 miles to get to the center of the earth. If you want to know what the inner core looks like, look up pictures of "iron meteorites". These meteorites come from the cores of asteroids and other bodies in our solar system that were smashed to bits when the solar system was forming.