What is the evidence for our knowledge of the internal structure of the Earth? As we enter the 21st century, we begin to learn more and more about the composition of the Earth. Early predictions led to some rather strange and peculiar thoughts about what constitutes our Earth, but today scientists can be sure that the Earth is made up of what they think. Through experiments and other sources of information, a picture of what is actually out there becomes much clearer. 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 which completely covers the earth. 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 drilling that humans have dug there. The mines that have been dug go down and still yield valuable minerals that can be found just as close to the Earth's surface. The deepest goes about 3 km into the earth and the temperature is 70¢XC, the only way the miners work is due to air conditioning, and yet the rock type is the same everywhere. Additionally, boreholes that have drilled halfway through the Earth's crust bring out rocks that closely resemble those on the surface. Scientists can therefore say with certainty that the Earth has a crust that is very similar in composition 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, traveling away from a seismic event. Primary waves are longitudinal and can pass through solids, liquids and gases. Secondary waves (S waves) propagate more slowly than primary waves and are transverse waves. This type of wave can only pass through solids. Measuring these waves is called seismology. Scientists have long known that the lava that comes out of volcanoes when they erupt comes from the mantle. The asthenosphere is the likely source of much basaltic magma, because the speed of S waves is slowed and partially absorbed in the asthenosphere..