The lure of the deep is driven by technological advances that make previously inaccessible oil now reachable, and dwindling supplies at shallower depths due to years of exploration. High energy prices and lucrative government incentives have also made it more financially feasible.
"That's where the oil is," said Eric Smith, associate director of Tulane University's Energy Institute. "You can't find any oil any cheaper anywhere else."
But the work is highly technical. In mile-deep seas, where BP PLC's Deepwater Horizon rig was plumbing for oil in an ocean canyon, the work is done in total darkness and near-freezing temperatures. Water pressure is enough to crush a submarine, and the explosive methane gas that likely ignited on the Deepwater Horizon can be much more damaging if not properly controlled.
More on the Gulf Disaster:
Probe: BP Oil Well Device Failed Safety Test
2nd Oil Containment Box Reaches Gulf Seafloor
Obama Seeks Greater Liability from BP on Spill
Fla. Gov. Crist Wants $35M from BP for Tourism
6 Dead Dolphins Wash Ashore; Oil's Role Unclear
Gulf Blame Game
"Drill, Baby Drill!" No More?
BP Oil Spill: Who's to Blame?
"The deep water is way too risky," said Kieran Suckling, executive director of the Center for Biological Diversity, a legal advocacy group that opposes such drilling.
A newhas found, meanwhile, that the general mood of Americans seems to be shifting against new drilling for oil and natural gas off the U.S. coastline.
Forty-six percent of Americans say they still support offshore drilling in the wake of the Gulf rig explosion - a 16-point drop from the 64 percent who backed such drilling in July of 2008, when "drill, baby, drill" was an oft-chanted Republican campaign slogan.
The Gulf of Mexico oil is a crucial part of the U.S. energy supply. While land-based production has steadily declined, output from the Gulf has doubled over the past two decades, surpassing that of any state including Texas and Alaska.
More companies are developing projects in mile-deep seas, and most of the oil produced in the Gulf now comes from sites more than 1,000 feet below the water's surface.
Drilling applications approved by the federal government last year went to wells at an average depth of 2,114 feet - triple what was seen just a decade ago, according to an Associated Press analysis of data from the federal Minerals Management Service.
This expansion of drilling ever deeper was fueled by the 1995 Deep Water Royalty Relief Act, which exempts companies from paying a percentage of royalties to the government on oil and gas found.
"Prior to that, the Gulf was becoming a dead sea in terms of exploration activities," said David Dismukes, director of policy analysis at Louisiana State University's Center for Energy Studies. "The relief act stimulated a lot of activity ... and concerns about environmental safety in the Gulf."
Politically, U.S.-produced oil from the Gulf of Mexico is an easier sell than continued reliance on foreign sources or tapping pristine Alaskan wilderness.
"In terms of oil that's available to us, it's either imported from places where it's bountiful," Smith said, "or we find deep water deposits off the coast of Africa, Brazil and the Gulf of Mexico."
That is widely expected to continue despite the Deepwater Horizon disaster, which killed 11 workers and has already fouled the Gulf with 4 million gallons of crude.
"This country's pretty well committed to oil and gas, and nobody's really happy when the price goes through the roof," said Philip Johnson, a petroleum engineering professor at the University of Alabama. "My suspicion is that we're going to proceed with production out of the Gulf of Mexico - and that means we're going to keep trying to get farther and farther offshore."
But the conditions down there have challenged oil companies to come up with technological solutions.
Beyond about 1,500 feet, sunlight is unable to penetrate. The temperature at 5,000 feet in the Gulf is about 40 degrees. The water pressure is about 2,300 pounds per square inch, conditions that can damage hydraulic systems or cause leaks.
A remotely operated underwater vehicle designed for 5,000 feet of water is "a whole world of different engineering" than one used at 500 feet, said Louisiana State University oceanographer Robert S. Carney. Yet companies must rely on those vehicles to detect or repair leaks and other problems.
Blowouts like the one that hit the Deepwater Horizon are also more powerful at extreme depths. They can thrust gas, oil, water and debris toward the surface with incredible force.
In the intense pressure and cold of the deep, methane hydrates exist in a slushy, crystalline form. But as methane rockets upward in a blowout, passing into lower-pressure zones, it converts to a gaseous state and gains tremendous force.
The use of heat in cementing, or sealing a well, which was under way prior to the blast, can destabilize methane hydrates at extreme depths. Halliburton Co., which was doing the cementing of the Deepwater Horizon well, acknowledged as much in an industry presentation last year, calling the risks "a challenge to the safety and economics."
Ultra deepwater rigs like Horizon are also required to have more robust blowout preventers that can withstand the intense pressure. The rig's preventer was designed to withstand 15,000 pounds of pressure per square inch, a standard for drilling at these depths. At an oil industry conference in Houston last week, there was an advanced 20,000-pound model on display.
Investigators trying to pinpoint a cause of the explosion are looking at both the blowout preventer and Halliburton's cementing. Interviews with rig workers conducted as part of BP's internal investigation indicate a bubble of methane escaped from the well and rocketed up the drill column, expanding as it approached the surface.
Transocean Ltd., which owned the rig and leased it to BP, is the major player in deepwater drilling, which 40 rigs worldwide working 5,000 feet down or greater. It said in a statement that the company's first commitment is safety.
"The company continues to strive and apply its core safety tools consistently across all its operations around the world," Transocean said.
The ongoing struggle to cap the leak also underscores the challenges when things go wrong far below the surface. BP's initial attempt to cap the underwater gusher with a 100-ton house-sized box designed to funnel the oil to a tanker ship was scrapped due to the formation of an icy slush that clogged it.
Company officials are now focused on positioning a smaller containment vessel, known as a Top Hat, or plugging a portion of the leak with junk. Those methods are long shots, however, never tried at such depths, and it could be up to three months before a relief well shuts off the leak entirely.
"Think of this same thing happening at less water depth: We could have easily sealed that well," said Satish Nagarajaiah, a Rice University engineering professor who focuses on offshore structures. "Now the challenges are bigger."
As oil companies continue to push the boundaries into new frontiers, they encounter unknowns: different sands, different hydrocarbons, different gas pressures. There's less of a history that can be used to assess problems that drilling may encounter.
"There's a real change when you go from conventional deepwater to much greater depths," said Tyler Priest, a University of Houston professor and expert on the history of offshore petroleum. "There are unknowns."