The Hubbert Peak theory posits that for any given geographical area, from an individual oil field to the planet as a whole, the rate of oil production tends to follow a bell-shaped curve. Early in the curve (pre-peak), production increases due to the addition of infrastructure. Late in the curve (post-peak), production declines due to resource depletion. The Hubbert curve satisfies these constraints. Furthermore, it is symmetrical, with the peak of production reached when half of the oil that will ultimately be produced has been. It also has a single peak.