The Earth system extends from the uppermost ionosphere to the innermost solid core of the planet and exhibits a wide variety of phenomena, such as climate change, ocean circulation, earthquakes, and geomagnetism. A vast number of parameters and precise models are needed to understand these complicated phenomena, which can significantly affect human society. Until recently, even the state-of-the-art supercomputers were not sufficiently powerful to run realistic numerical models of these phenomena. The Earth Simulator has been designed to provide a powerful tool for investigating these phenomena numerically with unprecedented resolution. It was built in 2002 as a collaborative project of the National Space Development Agency of Japan (NASDA), Japan Atomic Energy Research Institute (JAERI), and Japan Agency for Marine-Earth Science and Technology Center (JAMSTEC).