  We experimentally study the dynamics of quantum knots in a uniform magnetic
field in spin-1 Bose-Einstein condensates. The knot is created in the polar
magnetic phase, which rapidly undergoes a transition towards the ferromagnetic
phase in the presence of the knot. The magnetic order becomes scrambled as the
system evolves, and the knot disappears. Strikingly, over long evolution times,
the knot decays into a polar-core spin vortex, which is a member of a class of
singular SO(3) vortices. The polar-core spin vortex is stable with an observed
lifetime comparable to that of the condensate itself. The structure is similar
to that predicted to appear in the evolution of an isolated monopole defect,
suggesting a possible universality in the observed topological transition.
