The day after massive Hurricane Dennis churned through the Gulf of Mexico in July 2005, a commercial vessel traveling past BP PLC's hulking Thunder Horse oil platform radioed the bad news to its owner: The platform's top deck was listing into the water.

When a landing party from BP arrived at the platform two days later, they had to tie onto rails near the control tower to haul themselves up the platform's 30-degree incline.

"It looked like a ship that had been sunk," recalled Stan Bond, BP's head of subsea operations for the Gulf of Mexico.

The workers were surprised to learn that the platform, evacuated before Dennis hit, had not taken on water from a leak through its hull. Rather, an incorrectly plumbed, 6-inch length of pipe had allowed water to flow freely among several ballast tanks. That began a chain of events that caused the platform to tip into the drink.

Now BP is attempting to do what no oil company has done before: essentially rebuild the entire architecture of an oil field on the sea floor some 6,000 feet beneath the waves.

At $250 million, the job is costlier, and riskier, than putting the equipment on the gulf floor in the first place. On the frontier of oil exploration, the margin between riches and disaster can be as small as a 6-inch piece of pipe. Yet for BP, rebuilding the platform is critically important because the company desperately needs the oil flowing as reserves in formerly rich fields such as Prudhoe Bay in Alaska dwindle.

Politics have made oil from the Middle East, Africa, Russia and South America is increasingly out of reach. And new discoveries around the world are more rare and continue to shrink in size.

"We have passed the peak for world discoveries," said Robert Gillon, an analyst at oil-industry research firm John S. Herold Inc. "It's hard to see how the industry can do anything whatsoever to materially increase its oil reserves or production."

Against this backdrop, Thunder Horse, sitting atop a reserve that possibly holds 1.5 billion barrels, promises to deliver up to 250,000 barrels of oil a day, making it one of the gulf's biggest producers (this sentence as published has been corrected in this text). For U.S. consumers now paying an average of $3.10 a gallon for gas, Thunder Horse would relieve some of the price pressure: Fully operational, it would boost total U.S. production by 5 percent.

For BP, the troubles at Thunder Horse have turned the oil platform into a dual symbol. Like Janus, the two-faced Roman god that glimpses both the past and the future, Thunder Horse stands as a reminder of BP's mistake-prone recent track record. Looking forward, though, it holds out the prospect of a lucrative, rewarding future.

The Thunder Horse mishap followed by nearly four months BP's worst-ever accident on U.S. soil, a refinery explosion in Texas City, Texas, that killed 15 people. Then, last spring, BP spilled 200,000 gallons of oil onto the Arctic tundra, the first of several pipe leaks that ultimately led BP to temporarily shut down half of North America?s largest oil field (this sentence as published has been corrected in this text).

Yet getting Thunder Horse on line will not be easy. The deep Gulf of Mexico is challenging in its own right. Removing and reassembling an oil field at such depths has never been attempted.

One daunting challenge: delicately lifting miles-long strings of steel pipe from the sea floor. If the pipes stress or twist too much, they might weaken and perhaps spring a leak one day, resulting in disaster.

"It will not be easy to pull off," Bond said. "You're trying to change things to make something good. You've got to make sure you don't change things and make them worse."

Exploring the 'Dead Sea'

BP's history in the deep waters of the gulf began inauspiciously. Though the company had drilled in the shallows since the 1980s, in 1991 it began focusing on finding new reserves under water greater than 2,500 feet deep.

The early effort did not go well. At the outset, BP drilled a series of dry holes. At a cost well beyond $100 million, BP was learning why others in the industry had given the gulf a derisive nickname: "The Dead Sea."

"Drilling a succession of dry holes, it was almost the definition of insanity," said Cindy Yeilding, a leading geologist for BP's Gulf of Mexico effort. Executives at BP's London headquarters agreed. For two years, they would not approve any additional deep-water gulf drilling.

But that thinking changed. After a reassessment, BP's oil explorers decided on a new strategy that focuses all the company's energy on seeking big reserves, dubbed "elephants." And the company put big resources behind the new approach: as much as $2.5 billion annually in recent years on gulf exploration.