During its last 3-year run, the Large Hadron Collider (LHC) achieved its highest-energy collisions at 8 TeV. But when the LHC starts up again in 2015 it will hit 13 TeV, which means new challenges for the large detectors ATLAS, CMS, ALICE and LHCb. Subdetectors on the ATLAS experiment will have to be thoroughly tested for performance at high energy. But how do you test a general-purpose particle physics detector for high-energy collisions when there are no particle collisions taking place? "Cosmic rays," says ATLAS run coordinator Alessandro Polini.