In 2010, the Large Hadron Collider (LHC) at the laboratory CERN in Geneva, Switzerland, began its exploration of physics at distances 10,000 smaller than an atomic nucleus. The first data-taking period of the LHC ran from 2010 to 2013, colliding protons at energies of 7 and 8 TeV (trillion electron volts). Many novel observations were made, including the discovery of the long-sought Higgs boson. However, the most fundamental questions that motivated the LHC remain unanswered. The second data-taking period of the LHC began last summer, at the higher energy of 13 TeV.