"By location": A frequently used distinction considers whether processing is done above or below ground, and classifies the technologies broadly as "ex situ" (displaced) or "in situ" (in place). In "ex situ" processing, also known as above-ground retorting, the oil shale is mined either underground or at the surface and then transported to a processing facility. In contrast, "in situ" processing converts the kerogen while it is still in the form of an oil shale deposit, following which it is then extracted via oil wells, where it rises in the same way as conventional crude oil. Unlike "ex situ" processing, it does not involve mining or spent oil shale disposal aboveground as spent oil shale stays underground.