The rig, located about 50 miles off the Louisiana coast, erupted in flames when a concrete sealant failed and oil moved up piping from the ocean floor, about a mile beneath the surface of the water. The disaster claimed the lives of 11 workers and critically injured four more.
After burning for two days, the floating rig sank, buckling beneath it the piping connecting it to the oil well and causing at least three ruptures. A mechanism used to seal the well in the event of a blowout failed to activate, and crude oil has been gushing into the Gulf of Mexico at an estimated rate of 5,000 barrels, or 210,000 gallons, a day.
At least six million litres of petroleum have spilled so far, according to U.S. Coast Guard estimates, making it one of the worst U.S. oil spills in decades.
"The potential danger is unfathomable, because we don't yet know how the leak can be stopped and how big the spill will get," said Ken Rosenberg, director of conservation science at Cornell University. "It's a full moon, a high tide, and it's bringing the oil on a free ride right into the coastal salt marshes on a southerly wind."
Rosenberg said when the oil hits the shore, it would have an immediate impact on large numbers of birds, causing reproductive failure and possibly death.
"If the oil then comes into the coastal marshes and the inshore ecosystems and kills the oyster beds and the shrimp and the fish nurseries," he said, "then there are much longer-lasting effects not only on birds but on an entire way of life for people of this region."
Gov. Charlie Crist of Florida declared a state of emergency in several counties on Friday, saying the oil spill "threatens the state of Florida with a major disaster." Gov. Bobby Jindal of Louisiana declared a state of emergency in his state on Thursday.
Some officials worried the destruction could surpass the Exxon Valdez disaster 20 years ago. That oil tanker ran aground on the Bligh Reef in Alaska's Prince William Sound in 1989 and spilled 11 million gallons of crude oil.
BBC reported that the BP oil slick was roughly equivalent in size to Jamaica, and growing. It appeared set to continue washing ashore in Louisiana, and was likely to spread east along the gulf coastline in Mississippi, Alabama, and Florida.
Biloxi, Mississippi, Mayor A.J. Holloway compared the potential economic impact of the spill to the downfall that followed Hurricane Katrina in 2005.
"That was a tremendous blow to our city for several years and still has an impact," he told CNN Radio. "We just don't know what we can expect from this."
In a desperate battle now, booms are being deployed, dispersants sprayed, and some patches of oil are being burned. The Louisiana Coastal Protection and Restoration Authority is increasing the flow of water through two inland water channels in an attempt to wash oil-tainted seawater out of ecologically important wetlands.
"Home not only to a thriving fishing industry but also a substantial nature reserve, the potential for damage is enormous," says Simon Coxall from the UK's National Oceanography Centre in Southampton.
Booming the area off with floating dams to protect these areas is the best option; but the size of the spill will exhaust the world's supply of oil booms very quickly."
What makes this region ecologically special is the unusual patterns of land and sea conjured into existence by the lazy and variegated exit of the mighty Mississippi into the Gulf.
Here lie about 25% of US wetlands - areas rich with life, where human occupancy is low, and birds and other animals can thrive.
"For birds, the timing could not be worse; they are breeding, nesting and especially vulnerable in many of the places where the oil could come ashore," warns Melanie Driscoll, a Louisiana-based bird conservation director with the National Audubon Society, the leading US bird conservation group.
"We have to hope for the best, but prepare for the worst, including a true catastrophe for birds."
Efforts to contain the spill and prevent further oil from reaching the ocean surface have achieved little. Attempts to ring the slick with booms and administer controlled burns were hampered by wind and choppy seas.
The federal government is said to be heightening the pressure on BP, pushing the oil company to do more to stop well leaks gushing thousands of barrels of oil into the Gulf of Mexico and to beef up its response to the potential environmental impact on the coast.
"We'll continue to urge BP to leverage additional assets," U.S. Secretary of Homeland Security Janet Napolitano told reporters on Friday as the massive oil slick approached the Louisiana coast. "It is time for BP to supplement their current mobilization as the slick of oil moves toward shore."
EPA Administrator Lisa Jackson said there was a chance that workers will be able to stop the flow of oil into the Gulf of Mexico, but warned that the EPA is preparing for the worst.
"There is still the opportunity and the possibility that they would be able to shut it down," Jackson told CNN Chief National Correspondent John King. "Of course as responders we have to look at the worst case, and keep planning for that."
As fears rose -- particularly in the commercial fishing industry, a critical economic engine for the region -- President Obama promised steps to prevent a similar disaster in the future.
Speaking at the White House, the president said he had ordered Interior Secretary Ken Salazar "to conduct a thorough review of this incident and report back to me in 30 days on what, if any, additional precautions and technologies should be required to prevent accidents like this from happening again."
But he is seen to be pussyfooting on the issue. For he still maintains that domestic oil production continues to be "an important part of our overall strategy" but said "it must be done responsibly for the safety of our workers and our environment."
And only a month ago he had reversed a long-standing ban on off-shore drilling. "Expanding offshore drilling in areas that have been protected for decades threatens our oceans and the coastal communities that depend on them with devastating oil spills, more pollution
and climate change," lashed out Progress Florida's Mark Ferrulo.
Doug Suttles, chief operating officer of BP, said the company has had only three priorities since the April 20 rig explosion that led to the oil spill: stop the flow of oil, minimize its impact and keep the public informed.
"We've so far mounted the largest response effort ever done in the world," Suttles said at the same news conference. "We've utilized every technology available, we've applied every resource request. ... We welcome every new idea and every offer of support."
"Clearly, the sort of occurrence that we've seen on the Deepwater Horizon is clearly unprecedented," BP spokesman David Nicholas said Friday. "It's something that we have not experienced before ... a blowout at this depth."
But back in February 2009, the British energy giant had said it was "unlikely that an accidental surface or subsurface oil spill would occur" from the well it was proposing to drill 80 kilometres off the Louisiana shore.
And if such a spill did occur, the company said, "due to the distance to shore and the response capabilities that would be implemented, no significant adverse impacts are expected."
In hindsight, BP's 2009 government submission shows it failed to determine what kind of technology would be needed to control a possible spill at that depth.
The BP is attracting flak all round. Like Massey Energy, the company that refused to adopt safe mining practices at its Upper Big Branch coal mine and played cat-and-mouse games with federal regulators for years until 29 miners were killed in a mine explosion earlier this month, BP has an established track record of neglecting safety and proper maintenance, and it is no surprise that such practices have once again borne disastrous fruit, a commentator remarked.
In 2005, there was a fatal explosion at a BP refinery in Texas City. 15 people died and 170 were injured.
An official investigation into the causes of the Texas City explosion concluded in 2007 that senior BP executives, under the company's former chief executive, Lord Browne, had failed to act on red flags over safety at Texas City. Fatigue was a factor as one of the employees involved had worked 12-hour shifts for 33 consecutive days. And living quarters were positioned too close to safety-critical machinery.
In 1991, it was cited as the most polluting company in the U.S. Eight years later, it was fined $1.7 million for burning polluted gases at its Ohio refinery. In 2000, it paid another $10 million fine to the Environmental Protection Agency for air pollution.
It has also been revealed that the Obama administration had buckled before BP and oil industry pressure, failing to implement new safety and environmental regulations under consideration last fall. A BP executive wrote a letter to the administration stating that self-regulation was adequate.
Democratic Senators Bill Nelson of Florida and Frank Lautenberg and Robert Menendez of New Jersey called for hearings, saying the incident raises "serious concerns," over the industry's safety claims.
Climate-change legislation that expands offshore drilling will be tougher to pass "because of the fear and panic that has been engendered as a result of this catastrophe," Nelson told reporters in Washington.
Nelson and the other two senators have sent a letter to the chairmen and ranking members of the Commerce, Science and Transportation Committee and the Energy and Natural Resources Committee requesting a hearing on the incident. Since 2006, there have been 509 fires, at least two fatalities and 12 serious injuries on oil rigs in the Gulf of Mexico, according to the letter.
"This may be the worst disaster in recent years, but it's certainly not an isolated incident," the senators wrote. The explosion raises "serious concerns," over the industry's safety claims.
Obama proposed last month drilling for oil and gas off the U.S. East Coast and in the Gulf of Mexico 125 miles off the west coast of Florida. Obama said the plan is part of a transition to a new-energy economy that relies less on imported fossil fuel and more on domestic power from the sun and wind.
The explosion will force the industry to "at least admit that drilling for oil is a very risky, very dirty business," Michael Brune, executive director of San Francisco-based environmental advocacy group Sierra Club, said.
"There's no question that this complicates things a little bit because this plays into the argument from those that oppose offshore drilling," said Tom Moskitis, a spokesman for the American Gas Association, a Washington-based trade group. "It's going to make it harder to open more areas to production I'm afraid."