(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.
