Five years after the worst U.S. offshore oil spill, the industry is working on drilling even further into the risky depths beneath the Gulf of Mexico to tap massive deposits once thought unreachable.

But critics say energy companies haven't developed the corresponding safety measures to prevent another disaster or contain one if it happens — a sign, environmentalists say, that the lessons of BP's spill were short-lived.

The explosion aboard the Deepwater Horizon drilling rig engulfed the high-tech platform in flames, killed 11 men and injured hundreds. Oil, natural gas and toxic sludge poured into the Gulf for 87 days as regulators, industry and the White House struggled to contain the offshore disaster.

'It's not rocket science. Oh, no, it's much, much more complicated.'
- Matthew Franchek, University of Houston

Since the Macondo disaster five years ago Monday, federal agencies have approved about two dozen next-generation, ultra-deep wells.

The number of deepwater drilling rigs has increased, too, from 35 at the time of the Macondo blowout to 48 last month, according to data from IHS Energy, a Houston company that collects industry statistics.

Department of Interior officials overseeing offshore drilling did not provide data on these wells and accompanying exploration and drilling plans, information that The Associated Press requested last month.

But a review of offshore well data by the AP shows the average ocean depth of all wells started since 2010 is 40 per cent deeper than the average well drilled in the five years before that. And that's just the depth of the water.

'Golden zone'

Drillers are exploring a "golden zone" of oil and natural gas that lies roughly 6,095 metres (20,000 feet) beneath the sea floor, far deeper than BP's Macondo well, which was considered so tricky at the time that a rig worker killed in the blowout once described it to his wife as "the well from hell."

Geophysicists estimate oil companies can unleash Saudi Arabian-like gushers at these unprecedented depths from fields capable of yielding up to 300,000 barrels of oil a day.

Temperatures and pressures — the conditions that make drilling so risky — get more intense the deeper you go.

"It's not rocket science," said Matthew Franchek, director of the University of Houston's subsea engineering graduate program. "Oh, no, it's much, much more complicated."

Geoff Morrell, a BP spokesman, said his company has vastly improved its safety culture.

Several problems surfaced during the response five years ago. It became apparent that the Coast Guard and other federal agencies were heavily dependent on the industry for the equipment and expertise to cope with a deep-water blowout. Emergency plans on file were outdated and irrelevant.

A blowout in deeper water, farther from shore and containing even larger amounts of oil, would pose major challenges.

"We're setting the stage for the next Macondo blowout, and even worse," said Richard Charter, a senior fellow with the Ocean Foundation and a longtime industry watchdog.

The industry and regulators acknowledge the difficulties encountered at these new depths.

'Desperate need for energy'

"Going to greater depths, greater pressures, does present greater challenges," said Stephen Colville, president and CEO of the International Association of Drilling Contractors. "We have this desperate need for energy, and we have to go after it wherever it is."

BP PLC remains at the vanguard of ultra-deep exploration. Other companies, among them Chevron, Statoil, Shell, and Conoco-Phillips, are developing these deep fields, with the blessing of federal authorities.

"We believe absolutely that it is safe to drill these reservoirs," said Lars Herbst, the Gulf of Mexico regional director of the Bureau of Safety and Environmental Enforcement, the agency that oversees offshore drilling.

Since the BP spill, regulators and the industry publicly have said increasing safety is now a top priority. And they've taken a number of steps to make offshore drilling safer — including developing better cleanup equipment, hiring more inspectors, demanding tougher safety audits and setting better standards for drilling.

But critics say the improvements hardly go far enough and that engineering advances in drilling have far outpaced developments in safety and response technology.

The U.S. Chemical Safety Board recently found serious, even "fatal," design flaws with blowout preventers used throughout the Gulf.

"We will always push the frontier," said Ken Arnold, a petroleum engineer who served on the National Academy of Engineering panel that studied the Macondo disaster. "When I started in the industry, we seriously wondered if we could produce oil under 600 feet."