( CNN ) -- BP has said it hopes to have a relief well in place in August to permanently cut off the oil gushing into the Gulf of Mexico . The company has said the operation is the best sure-fire way of plugging the massive oil leak . A relief well is a well drilled into the existing well , intercepting the flow and allowing a specialized heavy liquid to be pumped into the flowing well to bring it under control . BP began drilling the first relief well on May 2 and started drilling a second one two weeks later . The wells are being dug to a depth of about 18,000 feet , just above the oil reservoir . Once one hits the original pipe , just 7 inches in diameter , drilling mud will be shot into it to stop the spewing oil . Cement will then be used to permanently seal it . The second relief well is part of a contingency plan in case the first well encounters any delays . Relief wells have been used to plug two other large blowout spills . In 1979 , it took nearly 10 months to seal the world 's largest spill in Mexico 's Bay of Campeche . More recently , an Australian accident last year known as the Montara spill , was plugged after nearly four months . In both cases , efforts such as those used to try to stop the Deepwater Horizon spill -- containment domes and junk shots -- were fruitless . And it took multiple tries before the relief wells reached the original wells .
Relief wells may be used to stop oil gushing into Gulf . BP hopes to have a well in place by August . Drilling for the first well began on May 2 .