Drilling for oil and natural gas involves penetrating numerous geological layers, many of which may be saturated with fresh or salt water, oil, natural gas, or combinations of all three.  To accomplish segregation between layers after a hole is drilled, steel casing is run into the bottom of the well and cemented in place.  Once the cement has hardened, all of the geological formations that have been penetrated are isolated from each other and the completion of the well can proceed.