Quantum Chromodynamics has long predicted a transition from normal hadronic matter to a phase where the quarks and gluons are no longer bound together and can move freely. There has been a worldwide hunt over several decades for this new phase, called quark gluon plasma. It is now produced regularly in collisions of heavy nuclei at very high energy at both the Relativistic Heavy Ion Collider (RHIC) in the U.S. and at the LHC in Europe. QCD plasma exhibits some remarkable properties. Its vanishingly small shear viscosity to entropy density ratio means that it flows essentially without internal friction, making it one of the most “perfect” liquids known. It is also very opaque to transiting particles, though the mechanism for this is not yet understood. The similarities to strongly coupled or correlated systems in ultra-cold atoms and condensed matter are striking, and have inspired novel theoretical descriptions growing out of string theory. It remains a big mystery how this plasma emerges from cold, dense gluonic matter deep inside nuclei. I will discuss how a future electron-ion collider can help address this question.