On 23 May 1984, a massive gas explosion ripped through the water valve house at Abbeystead outfall station in Lancashire, blowing its concrete roof into the air. The blast killed 16 and injured 26 visitors, many from the local village of St Michael's on Wyre who were packed in to watch a demonstration of the new facility.
The accident was caused by methane of geological origin leaking through the concrete tunnel walls in solution in water. When the pumps were restarted, air thick with methane was forced out, and ignited, possibly by a cigarette.
Poor design of the venting system, which fed into the valve house rather than to free air, was also blamed and the disaster led to revisions of regulations on confined spaces.