Almost a thousand years ago, in the arid climate of the southwestern United States, the Chacoan culture flourished. Ancestors of southwest Native American tribes today, Chacoans built impressive multi-storied stone buildings with a far-reaching trade system selling colorful macaws for turquoise. But a desperate lack of water—a megadrought—caused the advanced civilization to seemingly vanish within a generation. Described in a comprehensive new study published Wednesday in the journal Science Advances, scientists now understand the causes of the megadroughts common during the medieval period. With climate change, they predict more megadroughts in the future.