The Union process was an above ground shale oil extraction technology for production of shale oil, a type of synthetic crude oil. The process used a vertical retort where heating causes decomposition of oil shale into shale oil, oil shale gas and spent residue. The particularity of this process is that oil shale in the retort moves from the bottom upward to the top, countercurrent to the descending hot gases, by a mechanism known as a rock pump. The process technology was invented by the American oil company Unocal Corporation in late 1940s and was developed through several decades. The largest oil shale retort ever built was the Union B type retort.