The smaller the thing you look at, the bigger the microscope you need to use. That’s why the circular Large Hadron Collider at CERN, where they discovered the Higgs boson is 27 kilometres long, and its detectors tens of metres across. But to dig deeper still into the secrets of the Universe, they’re already talking about another machine 4 times bigger, to be built by the middle of the century. Roland Pease asks if it’s worth it.