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Abstract: Light takes just 3 yoctoseconds to traverse a proton—an ephemeral instant that nonetheless suffices to form a (locally) thermalized QCD medium from the initially out-of-equilibrium system right after a lead–lead collision at the LHC. Over the past two decades, experiments at CERN and Brookhaven National Laboratory have investigated the properties of this quark–gluon plasma, revealing it to be a nearly perfect liquid with the lowest viscosity ever observed in any known substance, and remarkably opaque to fast-moving colored particles. In this talk, I will review the current understanding of this system, with particular emphasis on the time evolution and physical mechanisms through which collective behavior emerges on such ultrashort timescales.