How did Earth get its water? It seems it had it all along. There are two possible sources for our water: either bombardment by meteorites soon after Earth’s formation, or it was present in the dust from which our planet formed. So a team from the University of Glasgow looked at the ratio of heavy hydrogen – an isotope known as deuterium – and normal hydrogen in water trapped for 4.5 billion years in volcanic rock. They found little sign of deuterium, which rules out meteorites since they have much more of this isotope. Instead the water must have originated in the dust cloud from which the solar system, including Earth, condensed.