In 1921, oil was discovered at the Long Beach Oil Field on and around Signal Hill. In 1932, the fourth-largest oil field in the United States, Wilmington Oil Field, was discovered; much of this field was underneath Long Beach and the harbor area itself. The hundreds of oil wells from Wilmington Oil Field provided oil revenues to the City and Port of Long Beach. The first offshore oil well in the harbor was brought online in 1937, shortly after the discovery that the oil field far extended into the harbor. In the mid-1930s, the port was expanded, largely due to the need to transport oil to foreign markets, as the immense output of oil from the Los Angeles Basin caused a glut in US markets.