The BP oil spill in the Gulf of Mexico was one of the world's largest oil leaks, taking almost three months to bring under control and leaking an estimated 5 million barrels of oil into the ocean.

Fire at Deepwater Horizon (Reuters)

It was BP’s fourth major incident in the United States in as many years.

The rig exploded on 20 April, killing 11 men. For months afterwards, oil belched out of the open well hole, 5,000 feet below sea level.

Initial estimations suggested that around 5,000 barrels a day were being released into the sea, however as the scale of the disaster became apparent those figures went up drastically to over 50,000 barrels a day.

The US government estimating about 206 million gallons of oil, nearly 5 million barrels in total, were released.

The volume of oil being released led experts to conclude that this spill could be worse than the Exxon Valdez spill of 1989, and could have far reaching consequences.

Desperate efforts were launched to try to protect the wildlife habitats along the US coastline, around the Mississippi Delta and President Barack Obama ordered a complete halt on oil exploration in the area.

Additionally, the White House suspended any new exploration in the Gulf pending the review of the explosion on the Deepwater Horizon oil rig.

Disaster sign (Getty)
6 May 2010: Louisiana oil spill 'an ecological time bomb'
"15 miles out from shore, the oil spill from BP's Deepwater well begins", writes Alex Thomson in Louisiana. "And as the oil enters the animal food chain, an ecological disaster looms."

Alex Thomson takes to the sea to witness the oil spill effects first hand. With him are Professor Rick Steiner and Greenpeace researcher Mark Floegel.

Professor Steiner explained that the booms laid out in the water are, in fact, incapable of collecting much of the oil because it has emulsified down into the water column.

Greenpeace researcher Mark Floegel told Channel 4 News we were no longer in the days of "drill, baby, drill" and were instead in the days of "spill, baby, spill".

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BP responsible for clean-up

The head of group media for BP, Andrew Gowers, told Jon Snow that the rig involved in the incident was operated by Transocean, an American company.

But he said it was BP's responsibility to cope with the consequences of the incident.

BP staged a massive clean-up operation, both below the surface in efforts to cap the well, and on the surface with the largest maritime clean-up the world had ever seen, with huge volumes of dispersants being sprayed on the leaked oil.

Gowers conceded that the oil spilling into the Gulf of Mexico was BP's – "and that's why we have primary responsibility for cleaning up the consequences and stopping it."

The Deepwater Horizon explosion

The Deepwater Horizon oil rig, which was manned by a crew of 126, burst into flames off the coast of Louisiana on Tuesday 20 April, killing 11 workers.

The rig was drilling BP plc's Macondo project with 126 workers on board when it ripped an explosion and fire ripped through it.

Fire engulfed the rig for nearly three days and still photographs show a structural collapse before it fell under the waves.

Specialised boats encircled the drowning rig after the explosion, spraying water in an attempt to contain the oil gushing from the crumbling structure, but neither the coast guard, nor BP or Transocean could approximate the scope of the damage immediately after the fire.

Officials from all three organizations hesitated to even say whether it was oil, gas or neither that was surging from the rig's uncorked seafloor well

The rig exploded almost three weeks after President Barack Obama unveiled plans for a limited expansion of US offshore oil and gas drilling.

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'Plug the damn hole'

The main focus following the disaster was on capping the leaking oil well on the sea bed, with BP trying several different methods to stem the flow.

Initial attempts all failed and frustrations mounted on all sides. On 26 May President Obama was said to demand of his team to "plug the damn hole".

Various different methods to plug the leak were attempted by BP - operations that would become characterised by their terminology: the "top hat", "top kill" and "junk shot".

A temporary containment cap was finally lowered over the leaking well on 13 July, which stopped the spread of oil into the Gulf. Although the flow of leaking oil was severely reduced, fears remained that the capping method itself was risking further ruptures elsewhere on the sea bed and a more permanent method was sought.

To this end, while these other operations were being conducted, BP had also began drilling two relief wells either side of the failed well.

These wells are due to intersect the original well in mid-August and pump cement into the broken well, finally seal the leak permanently.

Stopping the oil

Science Correspondent Julian Rush explains the options BP used to deal with the leaking oil.

First, in a bid to reduce the amount of oil floating on the surface so less comes ashore, they set light to some of it.

Oil that is at least several millimetres thick is corralled into a fire resistant boom, towed away from the spill, and then ignited, leading to what they call a "controlled burn" of several thousand gallons of oil lasting approximately one hour.

The burn could be stopped at any time by opening the boom and allowing the oil to spread too thin for combustion.

Second, efforts continued to activate the 'blow out preventer' on the well head. It consists of huge steel shutters that slice through the pipe to stop the flow. They were meant to be fail-safe: normally they are held open by huge hydraulic rams; in the event of a failure they should slam shut.

When the rig blew, it seems there was no time to operate them and no-one knows why they didn't activate automatically. Remote operated vehicles were used to try to operate them manually but with no success.

Next, BP could attempt to stem or "cap" the rate of flow from the seabed. Initial attempts to lower a steel "top hat" over the leak failed when methane build up threatened to life the hat off the leak.

Only when a containment cap was finally engineered and lowered onto the leak did the flow slow down in June. A second, more efficient cap was placed on the well head in July but again only as a temporary measure.
Finally there was the long term solution. Two new drilling sites were on site and began drilling relief wells. The aim was to drill down to meet the existing well in the seabed beneath the well head, to divert the flow and cap it again.

It's been done often enough before, but it's difficult - they have to drill down and sideways for several thousand feet to hit a target that is two feet wide, several hundred feet underground. And it is not a fast operation - the finish date is mid August.