  Monopoles and instantons are sheets (membranes) and strings in d=5+1,
respectively, and instanton strings can terminate on monopole sheets. We
consider a pair of monopole and anti-monopole sheets which is unstable to decay
and results in a creation of closed instanton strings. We show that when an
instanton string is stretched between the monopole sheets, there remains a new
topological soliton of codimension five after the pair annihilation, i.e., a
twisted closed instanton string or a knotted instanton.
