When a major earthquake strikes, there is usually a brief period of time when the earth moves, shifts, and even shakes. Although this is a natural part of life, it is not something you expect to experience in your lifetime. However, you might be surprised when you feel the ground beneath you move and shake. This is known as an earthquake. When this happens, it is not the earth that is moving, but the rocks within the earth. The most common effect is a change in the air pressure. This is why you can feel like the air is suddenly much lighter or heavier.