What evidence supports our knowledge of the internal structure of the Earth? As we enter the twenty-first century we are starting to learn more and more about the composition of the Earth. Early predictions brought out some rather strange and peculiar thoughts about what makes up our Earth, but today scientists can be certain that the Earth is made up of what they think. Thanks to experiments and other sources of information, the picture of what is really down there is becoming increasingly clear. So how do these scientists know that sections of the Earth are made up of different compositions, and how do we know that the physical state of each layer is what it is? The outermost layer of the Earth is the crust, it is what we stand on and it covers the earth entirely. It is made up of many different rocks and minerals, we know that the composition of the earth's crust is generally the same due to the mines and wells dug by humans. The mines that have been dug go down and still uncover valuable minerals that can be found right near the earth's surface. The deepest goes about 3km into the earth and the temperature is 70¢XC, the only way the miners work because of the air conditioning, yet the rock type looks the same everywhere. Wells drilled halfway into the Earth's crust also reveal rocks that look very similar to those on the surface. So scientists can safely say that the Earth has a crust that is very similar in composition all the way down to the mantle. When earthquakes occur, they produce two types of waves: P waves and S waves. Primary waves (p waves) are the fastest waves, moving away from a seismic event. Primary waves are longitudinal, they can travel through solids, liquids and gases. Secondary waves (s waves) travel slower than primary waves and are transverse waves. This type of wave can only travel through solids. The measurement of these waves is called seismology. Scientists have long known that the lava that comes out of volcanoes when they erupt came from the mantle. The asthenosphere is the likely source of much of the basaltic magma, this is because the velocity of S waves is slowed and partially absorbed into the asthenosphere.