The Large Hadron Collider (LHC) at CERN usually collides protons together. It is these proton–proton collisions that led to the discovery of the Higgs boson in 2012. But the world's biggest accelerator was also designed to smash together heavy ions, primarily the nuclei of lead atoms, and it does so every year for about one month. And for at least two good reasons. First, heavy-ion collisions at the LHC recreate in laboratory conditions the plasma of quarks and gluons that is thought to have existed shortly after the Big Bang. Second, the collisions can be used to test and study, at the highest manmade temperatures and densities, fundamental predictions of quantum chromodynamics, the theory of the strong force that binds quarks and gluons together into protons and neutrons and ultimately all atomic nuclei.