In cities all across the world, we go about our daily routines, feeling confident that our lives are orderly and predictable. But at any moment, that confidence can be shattered, as nature demonstrates that it still has the upper hand. When we least expect it, or when we’re least prepared, disasters can strike. Every day, the Earth is shaken by hundreds of small earthquakes. Most go unnoticed. They usually occur along the boundaries of the thin plates that cover the earth like an eggshell. Driven by the heat deep within the Earth's core, the plates grind against each other along lines, called faults. When the plates find their motion blocked, stress builds up. Finally the fault gives way. The released energy races through the Earth in the form of seismic waves. However, if we didn’t have earthquakes, the Earth would be a cold, dead place. If it wasn’t for this great flow of heat, there would be no continents, no oceans, and no atmospheres. The Earth would be as dead, dry, and cold as the Moon.