A plug (1) for plugging wells, and in particular oil and gas wells, is provided. The plug (1) has a plug body formed from an outer metal tube (2) of a reduced thickness. The plug also has reinforcement means(4), attached to an inner surface of the outer tube (2), that give the plug a cross-sectional structural strength that is at least equivalent to that of a thicker metal tube. The plug has a central heater receiving void located along the axis of the plug to enable a plug deployment heater to be received therein. Also provided is a plug assembly (10) with a variable cross-sectional area in a plane perpendicular to the plane in which the assembly is deployed during the plugging of underground conduits.