In the fall of 2008, CERN's high-energy physicists ran into a problem. A faulty electronic connection at the Large Hadron Collider in Switzerland—the biggest, baddest, most powerful particle accelerator ever built—caused a couple of magnets to overheat and melt, triggering an explosion of pressurized helium gas. The accident, which happened just nine days after the LHC turned on for the first time, led to months of delays. "It was pretty depressing when we broke the accelerator," says Aaron Dominguez, a physicist at the University of Nebraska. "That was not a good day."

Eventually, engineers fixed the LHC, and in 2012, physicists used it to do what the accelerator was always supposed to: Find the elusive subatomic particle called the Higgs boson. It worked, earning much fanfare and a Nobel Prize. But to prevent another accident, CERN's engineers had run the LHC at only half its designed capability. Now, after a two-year hiatus in which engineers upgraded the accelerator to prevent such magnetic meltdowns, the LHC is set to smash protons together harder than ever—the way it was intended. "It's like having a new accelerator, really," Dominguez says. The increased power will mean more violent collisions that might create bigger, even rarer particles.