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The number of crude oil and natural gas rotary rigs in operation in the United States has fluctuated greatly since the mid-20^th century. Oil production in the United States dropped steadily from the 1960s, as the OPEC bloc began producing and exporting oil at low prices, however the shocks of the 1970s saw oil prices rise significantly after OPEC placed an embargo on the U.S. and its allies in 1973. The U.S. ramped up production to try and negate some of the effects of this embargo, but the long term effect of this was that demand in the 1980s dropped, in what was known as the 1980s oil glut. The number of oil and gas rotary rigs then remained below 1,000 throughout the 1990s, before the oil boom in South Dakota and the move towards self-sufficiency took place in the early 2000s. In more recent years, rotary rigs are being replaced by top-drive drills, which are much more efficient as they can bore for two or three times more than a rotary rig in one section.