Magnetic monopoles are hypothetical particles proposed by physicists that carry a single magnetic pole, either a magnetic North pole or South pole. In the material world this is quite exceptional because magnetic particles are usually observed as dipoles, north and south combined. However there are several theories that predict the existence of monopoles. Among others, in 1931 the physicist Paul Dirac was led by his calculations to the conclusion that magnetic monopoles can exist at the end of tubes – called Dirac strings – that carry magnetic field. Until now they have remained undetected.