Check out this bizarre natural phenomenon. Make sure to play the movie.
Stable vortexes form at the ends of bends in rivers as the water that accelerates on the outside of a bend collides with the slower-moving water that traversed the inside of the bend. The velocity sheer produces a vortex. Apparently, an ice pan formed in the center of the stable vortex and then slowly grew until it reached the turbulent edge of the vortex. The turbulence at the edge prevented ice from forming leaving a lubricating liquid layer that lets the ice pan rotate in the vortex.