First came Holstein, then Mad Dog, and soon, Thunder Horse. Atlantis will join them next year. The four giant oil fields, operated by BP PLC (BP ) and located under thousands of feet of water off the coast of Louisiana, are just beginning to pump their first barrels. At their peak rates later in the decade, they'll produce some 500,000 bbl. per day, an amount akin to floating a small Middle Eastern country such as Syria or Yemen into the Gulf of Mexico. "Add them together, and it's a massive step change," says David Eyton, BP's vice-president for deepwater in...