Since the start of the ultra-relativistic heavy-ion collisions at RHIC (BNL) and most recently at LHC (CERN), the study of matter under extreme conditions has entered an unprecedented era. The primary goal of the heavy-ion program is to create and study a novel state of matter, namely the quark-gluon plasma (QGP), by smashing high energetic nuclei. Strikingly, the QGP created in experiments behaves like a strongly coupled liquid characterized by a tiny shear viscosity, rather than a weakly coupled gas as predicted by the fundamental theory QCD. To reconcile this apparent conflict between theory and experiment has been a key open question in the community since 15 years. In this talk, I will first give a heuristic introduction to this question. Then I will report a recent discovery of a novel massless collective excitation in the non-Abelian plasma which may provide a crucial ingredient in resolving this conflict. This new mode is generated by the confining (chromo)magnetic scale g^2T at high temperature, and it consequently induces positivity violation in the corresponding thermal spectral functions. This demonstrates the profound impact of confinement effects on collective excitations of the QGP, which manifest genuine long-range correlations that is necessary in rendering the system strongly coupled.