A method and apparatus for forming a plug in a passageway. The apparatus comprises a carrier which in use is lowered into the passageway, the carrier comprising an elongate body (3) of a material resistant to creep which supports at least two spaced apart portions (5) that are a sliding fit in the well (1) such that a gap is formed between each of the potions and the passageway. A body of material (15, 16, 17) the :melting point of which is higher than the temperature within the passageway and which expands as it solidifies is supported on the carrier. A heating device is provided for melting the body of material such that the melted material fills a space defined between the first and second portions. The carrier is cooled preferentially to ensure that the molten material solidifies rapidly adjacent the two spaced apart portions, thereby resisting flow of the material as it solidifies past the spaced apart portions. Means (21) are provided to obstruct the gaps formed between the portions arid the passageway; the obstructing means being displaced into the gaps as a result of melting of the body of material or as a result of any creep of material after it has been melted and solidified.