The hypothesis that Earth was completely covered in ice (Snowball Earth) 635 million years may not be so. The atmospheric concentration of carbon dioxide during that period was much lower than previously thought, according to a team of French researchers from the Institut de Physique du Globe de Paris (CNRS/IPGP/Université Paris Diderot), working in collaboration with scientists from Brazil and the U.S. The Snowball Earth hypothesis posits that the Earth’s surface became entirely or nearly entirely frozen at least once. Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical paleolatitudes, and other otherwise enigmatic features in the geological record.