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Crews dig about 20 to 30 feet at a time, then run electric current through the relief well. The current creates a magnetic field in the pipe of the blown-out well, allowing engineers to calculate exactly where and how far they need to drill.

The flow of oil into the Gulf has been halted since July 15, when a temporary cap over the well was able to contain the spill. But officials have stressed for weeks that only a bottom kill will ensure the well is no longer a danger.

Before July 15, the oil leaked almost unimpeded for nearly three months and spewed some 206 million gallons of oil, according to the government’s latest estimate. The crisis began on April 20, after an explosion on the BP PLC-leased Deepwater Horizon drilling rig that killed 11 workers. Officials don’t yet know the cause of the explosion, or why machinery designed to prevent the unchecked flow of oil failed to work.

BP has already spent $6.1 billion responding to the spill.

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Associated Press Writers Harry R. Weber and Noaki Schwartz contributed to this report from New Orleans.