Experiments at large particle colliders like RHIC at Brookhaven Lab and LHC at CERN have used collisions of heavy nuclei to study a simple question:
What are the properties of matter at extremely high temperature, in excess of several trillion Kelvin? At such temperatures, matter exists in the form of a plasma of quarks and gluons resembling the universe shortly after the Big Bang. We have found that the produced state of matter exhibits fascinating and somewhat surprising properties: Although its density exceeds that of water by 16 orders of magnitude, the quark-gluon plasma behaves like a near-perfect liquid. I will review the most striking experimental observations and describe new experiments aimed at elucidating the emergence of the plasma's unusual properties.