Introduction to Magnetic Monopoles
  One of the most basic properties of magnetism is that a magnet always has two poles, north and south, which cannot be separated into isolated poles, i.e., magnetic monopoles. However, there are strong theoretical arguments why magnetic monopoles should exist. In spite of extensive searches they have not been found, but they have nevertheless played a central role in our understanding of physics at the most fundamental level. 
Arttu Rajantie
hep-th hep-ph physics.pop-ph