A lighted match is dropped by a smoker down a gap in the wooden escalator at the King's Cross tube station. It ignites wax under the escalator. When someone notices the glowing fire under the escalator, the person stops the escalator. Later, a small flame rises on the steps. The full extent of the fire is hidden under the escalator. The entire area under it is on fire and the escalator aboves heats up. It gets so hot that is combusts and blasts a fireball up the escalator and kills 31 people in the ticket hall 20 metres away. The effect that happened is now called the trench effect.
The King's Cross fire began at approximately 19:30 on 18 November 1987 at King's Cross St Pancras tube station, a major interchange on the London Underground. As well as the mainline railway stations above ground and subsurface platforms for the Metropolitan, Circle and Hammersmith & City lines, there were platforms deeper underground for the Northern, Piccadilly, and Victoria lines. The fire started under a wooden escalator serving the Piccadilly line and, at 19:45, erupted in a flashover into the underground ticket hall, killing thirty-one people and injuring 100.

A public inquiry was conducted from February to June 1988. Investigators reproduced the fire twice, once to determine whether grease under the escalator was ignitable, and the other to determine whether a computer simulation of the fire—which would have determined the cause of the flashover—was accurate. The inquiry determined that the fire had been started by a lit match being dropped onto the escalator. The fire seemed minor until it suddenly increased in intensity, and shot a violent, prolonged tongue of fire, and billowing smoke, up into the ticket hall. This sudden transition in intensity, and the spout of fire, was due to the previously unknown trench effect, discovered by the computer simulation of the fire, and confirmed in two scale model tests.

London Underground was strongly criticised for its attitude toward fires; staff were complacent because there had never been a fatal fire on the system, and had been given little or no training to deal with fires or evacuation. The report on the inquiry resulted in resignations of senior management in both London Underground and London Regional Transport and to the introduction of new fire safety regulations. Wooden escalators were gradually replaced with metal escalators on the Underground.
King's Cross St Pancras tube station has subsurface platforms for the Metropolitan, Circle, and Hammersmith & City lines. Deeper underground are the platforms for the Northern line City branch and the Piccadilly and Victoria lines. An escalator shaft led down to the Victoria line and another led down to the Piccadilly line, and from that to the Northern line. Stairs connected the Piccadilly and Victoria line platforms and from these there was a subway to King's Cross Thameslink railway station platforms used by British Rail Midland City (later Thameslink) trains