  We experimentally and theoretically explore the creation and time evolution
of vortex lines in the polar magnetic phase of a trapped spin-1 $^{87}$Rb
Bose-Einstein condensate. A process of phase-imprinting a nonsingular vortex,
its decay into a pair of singular spinor vortices, and a rapid exchange of
magnetic phases creates a pair of three-dimensional, singular singly-quantized
vortex lines with core regions that are filled with atoms in the ferromagnetic
phase. Atomic interactions guide the subsequent vortex dynamics, leading to
core structures that suggest the decay of the singly-quantized vortices into
half-quantum vortices.
