The Giant Magellan Telescope Organization (GMTO) announced that it has initiated the casting of the fifth of seven mirrors that will form the heart of the Giant Magellan Telescope (GMT). The mirror is being cast at the University of Arizona’s Richard F. Caris Mirror Laboratory, the facility known for creating the world’s largest mirrors for astronomy. The 25-meter diameter GMT will be sited in the Chilean Andes and will be used to study planets around other stars and to look back to the time when the first galaxies formed. The process of “casting” the giant mirror involves melting nearly 20 tons of glass in a spinning furnace. Once cooled, the glass disk will be polished to its final shape using state-of-the-art technology developed by the University of Arizona.