According to the established theory of the strong interaction, quantum chromodynamics (QCD), quarks and gluons are the fundamental building blocks from which atomic nuclei are assembled. The quark-gluon deep structure of nuclear matter is well concealed, however. Quarks and gluons are tightly packaged into protons and neutrons, which form the more conspicuous units. But theory predicts that under extreme conditions of high temperature or high density the physical nature of this matter changes drastically: Protons and neutrons dissolve. Quarks and gluons then become the obvious as well as the fundamental units of description. Physicists therefore speak of “quark-gluon matter.”