  The quark-gluon plasma produced by collisions between ultra-relativistic
heavy nuclei is well described in the language of hydrodynamics. Non-central
collisions are characterized by very large angular momentum, which in a fluid
system manifests as flow vorticity. This rotational structure can lead to a
spin polarization of the hadrons that eventually emerge from the plasma,
providing experimental access to flow substructure at unprecedented detail.
Recently, first observations of $\Lambda$ hyperon polarization along the
direction of collisional angular momentum have been reported. These
measurements are in broad agreement with hydrodynamic and transport-based
calculations and reveal that the QGP is the most vortical fluid ever observed.
However, there remain important tensions between theory and observation which
might be fundamental in nature. In the relatively mature field of heavy ion
physics, the discovery of global hyperon polarization and three-dimensional
simulations of the collision have opened an entirely new direction of research.
We discuss the current status of this rapidly developing area and directions
for future research.
