The relief well

The relief wells drilled into the blown-out Ixtoc well in Mexico three decades ago took about three months to quell the gusher. Engineers in the current spill are confident they can pull it off by August, although some are skeptical.

"The petroleum engineers seem pretty cocky about that," Ed Overton, a Louisiana State University professor of environmental studies. "It just strikes me that there are so many unknowns. My guess is that it's going to take more than one try."

The digging is a trial-and-error process. As the drill plunges deeper through the Earth's crust, crews will probe the area with a high-tech metal detector to guide their way to the shaft. If they punch it too far, engineers will have to reverse and then plug the hole with cement. Experts say each miss could take days or longer to fix.

"In order to do this perfectly you'd have to know exactly where the original well is," said David Rensink, incoming president of the American Association of Petroleum Geologists. "And unfortunately the directional data that goes into this is not perfect."

Associated Press Writer Ray Henry contributed to this report.

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