In the summer of 1985 a severe hydrogen-air explosion occurred in an ammonia plant in Norway. Theaccident resulted in two fatalities and the destruction of the building where the explosion took place. Thispaper presents the main findings from an investigation in 1985 and 1986 of the gas explosion and itsconsequences. The event started when a gasket in a water pump was blown out. The water pump wassituated inside a 100 m long, 10 m wide, and 7 m high building. The pump was feeding water to a vesselcontaining hydrogen gas at pressure of 30 bars. This pressure caused a back flow of water flow through thepump and out through the failed gasket. The hydrogen reached the leakage point after about 3 minutes. Thedischarge of gas lasted some 20 to 30 seconds before the explosion occurred. The total mass of the hydrogendischarge was estimated at 10 to 20 kg hydrogen. The main explosion was very violent and it is likely thatthe gas cloud detonated. The ignition source was almost certainly a hot bearing. Several damage indicatorswere used to estimate the amount of hydrogen that exploded. The indicators include deflection of pipes andpanels, distances traveled by fragments, and the distribution of glass breakage. We found that 3.5 to 7 kg ofhydrogen must have been burning violently in the explosion. Window glass was broken up to 700 m fromthe centre of the explosion. Concrete blocks, originally part of the north wall of the building and weighing1.2 metric tons were thrown up to 16 meters. The roof of the building was lifted by an estimated 1.5 metersbefore resettling. The displacement of the roof caused a guillotine break of a 350 mm diameter pipeconnected to the vessel that was the source of the original gas discharge. The gas composition in the vesselwas 65 - 95 %2hydrogen. This resulted in a large horizontal jet fire lasting about 30 seconds. Minorexplosions occurred in the plant culvert system.To our knowledge this gas explosion is one of the largest industrial hydrogen explosions reported. Webelieve this case history is a valuable reference for those who are investigating the nature of accidentalhydrogen explosions.