The quark-gluon plasma is the first fundamentally new state of matter to be discovered since atoms were first split into nuclei and their electrons (a regular plasma) at the start of the last century. In this state, which is likely formed briefly in high-energy cosmic-ray collisions on a regular basis, but hasn’t been widespread since the Big Bang, protons and neutrons dissolve into their consituent quark components, along with the color force “gluons” that usually bind nucleons together. The strong theory of matter describes this state reasonably well in simulation, but it has been quite difficult to achieve experimentally.
Of course, back when RHIC was first being opened a few years ago, people were very worried about recreating the Big Bang… luckily disaster has not yet struck Long Island.