Before the Deep Underground Neutrino Experiment (DUNE) comes on line, the smaller two-detector protoDUNE program at CERN will test the underlying design. Seen here is the inside of one of protoDUNE’s liquid-argon time-projection chambers; special lights that protect the photon-detection elements impart a gold hue. On page 46 , Anne Heavey explores the numerous challenges to making the world’s largest particle detector a reality. (Courtesy of Maximilien Brice/CERN.)