Abstract: The invention concerns a method for establishing a plug inside a main pipe, in particular a vertically or slantingly arranged pipe, where said pipe has firs and second ends, and where the plug is arranged inside the pipe at a distance from said first and second ends, where a liquid surface is provided at least inside the pipe, below the level at which it is desirable to establish the plug, where a buoyant layer is established on top of said liquid surface, and where a plug material is placed on top of ‘said buoyant layer as well as a plug established according to the method. The buoyant layer can be granular, liquid or solid material. The plug is made of foam material, expandable material or cement-based material such as concrete.