elephant
 n.— «Though more than a thousand discoveries are made in the U.S. yearly, “elephant fields”—those containing proven recoverable reserves of 100 million barrels of oil, one trillion cubic feet of gas, or the equivalant in mixed reserves—are rare. » —“Hottest U.S. Hydrocarbon Hunt Is In Western Rock Formation” Wall Street Journal Aug. 27, 1979. (source: Double-Tongued Dictionary)