This article, from Earth: Inside and Out, takes a look at the widespread influence of the Nobel Prize-winning chemist Harold C. Urey. It covers his discovery of deuterium and the large-scale process he devised for obtaining water with high proportions of this isotope. The role of Urey as director of the Atomic Bomb Project at Columbia University, along with his contributions to the atom bomb and the H-bomb is also covered. Using oxygen isotopes as a paleothermometer, Urey and his co-workers analyzed tiny fossil shells from ocean sediment cores to create a history of changing ocean temperatures over long geologic periods. This was how Urey used his knowledge of isotopes to calculate the temperature of ancient oceans. Urey also studied the chemical make-up of the Sun, Moon and planets, and formulated detailed theories about the origin of the solar system. He believed that planets were built up by the accretion of smaller, mainly metallic fragments at relatively low temperatures, and that the Moon was formed separately.