A hopfion is a topological soliton. It is a stable three-dimensional localised configuration of a three-component field n → → = ( n x , n y , n z ) with a knotted topological structure. They are the three-dimensional counterparts of 2D skyrmions, which exhibit similar topological properties in 2D. Hopfions are widely studied in many physical systems over the last half century, as summarized here http://hopfion.com