Methane gas detected in and around properties in a residential area of St Helens could have originated from one of several sources: gas distribution mains, faulty sewers, shallow coal seams and a nearby landfill site. Investigations eliminated some of these, and a landfill source was eventually confirmed by radiocarbon dating. Drilling, together with the records of an abandoned opencast prospect, showed that the gas was migrating through ancient shallow coal workings, possibly reaching the surface via a fault zone. Temporary remedial works were carried out to ventilate the mine workings in the residential area. The permanent remedy consisted of grouting the workings along the boundary of the landfill site to prevent further lateral migration, together with on-site methane ventilation.