: The Tibetan Plateau is an area one-fourth the size of the US with an average altitude of 5 km. Bordered to the south by the Himalayan mountains, the area is thought to have begun to rise about 50 million years ago, as the Indian continent collided with Eurasia. A new study of oxygen isotopes in rocks from the Linzhou Basin suggests that the area may have reached its current elevation before the collision period began. Ding Lin of the Chinese Academy of Sciences’ Institute of Tibetan Plateau Research in Beijing and his colleagues examined rocks dating to 55 million years ago and compared the ratio of oxygen-18 to oxygen-16. Oxygen-18, being heavier, is more likely to precipitate in larger quantities at higher elevations, and the ratio of the isotopes allows for a calculation of altitude. Geological processes in which the rocks get heated and compressed could have altered the ratio, but the researchers say that the presence of certain fossils indicates that the rocks are unaltered.