The Hubbert peak theory is based on the observation that the amount of oil under the ground in any region is finite, therefore the rate of discovery which initially increases quickly must reach a maximum and decline. In the US, oil extraction followed the discovery curve after a time lag of 32 to 35 years. Peak oil theory is based on the observed rise, peak, fall, and depletion of aggregate production rate in oil fields over time. It is often confused with oil depletion; however, whereas depletion refers to a period of falling reserves and supply, peak oil refers to the point of maximum production. In theory, peak oil can be brought on by the production squeeze — the drawdown as adding new reserves gets more challenging — but it can also be caused by a production decline when oil alternatives become more cost-effective, pricing oil out of the market, and making exploration and production unprofitable.