Reserves refer to crude oil or natural gas deposits that remain underground and have not yet been extracted. The three "P"s of reserves refer to "proven," "probable," and "possible." These correspond with the likelihood of successfully drilling in those deposits. The oil& gas industry gives proven reserves a 90% certainty of being produced (also known as P90). The industry gives probable reserves a 50% certainty (P50), and possible reserves just a 10% certainty (P10) of actually being produced.