  Magnetic monopoles --- particles that behave as isolated north or south
magnetic poles --- have been the subject of speculation since the first
detailed observations of magnetism several hundred years ago. Numerous
theoretical investigations and hitherto unsuccessful experimental searches have
followed Dirac's 1931 development of a theory of monopoles consistent with both
quantum mechanics and the gauge invariance of the electromagnetic field. The
existence of even a single Dirac magnetic monopole would have far-reaching
physical consequences, most famously explaining the quantization of electric
charge. Although analogues of magnetic monopoles have been found in exotic
spin-ices and other systems, there has been no direct experimental observation
of Dirac monopoles within a medium described by a quantum field, such as
superfluid helium-3. Here we demonstrate the controlled creation of Dirac
monopoles in the synthetic magnetic field produced by a spinor Bose-Einstein
condensate. Monopoles are identified, in both experiments and matching
numerical simulations, at the termini of vortex lines within the condensate. By
directly imaging such a vortex line, the presence of a monopole may be
discerned from the experimental data alone. These real-space images provide
conclusive and long-awaited experimental evidence of the existence of Dirac
monopoles. Our result provides an unprecedented opportunity to observe and
manipulate these quantum-mechanical entities in a controlled environment.
