High energy physics experiments have shown that particle emission in a wide variety of reactions can be well describe by thermal models. This effect is being used to map out the phase boundary between the hadronic and the quark-gluon phase of strongly interacting matter. Experiments at the Relativistic Heavy Ion Collider will soon probe new domains of this phase diagram. My lecture will focus on novel insight into the properties of hot quark-gluon matter and on new ideas that may help explain its surprisingly thermal behavior on the basis of chaos theory. The colloquium will - hopefully - explain what makes the (heated) QCD vacuum such a special and unusual physical state.