On April 20, 2010, the explosion on the Deepwater Horizon drilling rig in the Gulf of Mexico led to the largest accidental release of oil into marine waters in history.

BP was using the rig to drill an exploratory well in the Macondo field, 66 kilometres from the Louisiana coast. Eleven workers on the rig died, 17 were injured. Two days later, on Earth Day, the rig sank.

The leak was finally capped on July 15. Almost five months after the explosion, the official in charge of the spill response, retired U.S. Coast Guard admiral Thad Allen, said, "We can now state definitively that the Macondo well poses no continuing threat to the Gulf of Mexico."

On March 30, people familiar with the probe into the spill said U.S. Justice Department investigators are exploring possible charges of manslaughter and perjury against companies or managers responsible for the explosion that killed 11 workers.

As of March 31, there were still 184 vessels and 2,444 personnel working on cleaning up the spill.

Size of the oil spill: 5 million barrels

An exhausted, oil-covered brown pelican sits in a pool of petroleum along Queen Bess Island Pelican Rookery in Louisiana on June 5. Sean Gardner/Reuters

On Oct. 6, the National Oil Spill Commission reported, "The emerging consensus is that roughly five million barrels of oil were released by the Macondo well."

That amount is equivalent to about half of the crude oil the U.S. imports in a day and worth about $400 million US.

On Aug. 4, the U.S. government released a report on the fate of the oil released. In the report, the commission was critical of the government for both underestimating the amount of oil that remained in the Gulf and for presenting the budget as a scientific assessment, rather than "a rough operational tool." The "findings were neither as clear nor as reassuring as the initial rollout suggested," the commission wrote.

It singled out Carol Browner, the White House co-ordinator for energy and climate change, for stating on Aug. 4 that "more than three-quarters of the oil is gone." The oil that was dissolved or dispersed was not "gone," but is potentially being biodegraded.

In August, Samantha Joye, a marine scientist from the University of Georgia, found a layer of oily substance up to five centimetres thick covering the ocean floor in the region of the BP well. A team from the University of South Florida made similar findings.

Contaminated area

About 1,070 kilometres of Gulf of Mexico coastline were contaminated by the oil.

The area closed to fishing reached a peak 225,290 square kilometres on June 21. That was the 18th expansion of the closure area of Gulf of Mexico federal waters, about 36 per cent of the total area.

Response

• More than 47,000 personnel deployed
• 3,474 kilometres of containment and absorbent boom were deployed against the spreading oil slick on Aug. 2
• 411 in-situ burns conducted (which burned 265,450 barrels of oil)
• 1.8 million gallons of dispersants used (French biologist Philippe Bodin, who studied the effects of the Amoco Cadiz spill in 1978, found the dispersant to be more toxic to marine life than the oil and told National Geographic magazine he fears its massive use this time will be "catastrophic for the phytoplankton, zooplankton and larvae." BP claims the dispersant is no more toxic than dishwashing soap.)
• 1.4 million barrels of liquid waste collected
• The total cost of the clean-up has been estimated at more than $40 billion US

Fish and wildlife

• The latest report from the oil spill's Unified Area Command shows that 2,080 oiled birds were collected alive. About 1,245 of those birds have been released.
• 6,104 dead birds were also collected.
• 535 sea turtles were collected alive, 397 released.
• 609 dead turtles have been collected.
• 278 sea turtle nests were transported.

Gulf of Mexico oil industry

• Thirty per cent of U.S. oil production comes from the Gulf of Mexico.
• Oil production in the Gulf increased 34 per cent in 2009.
• The Gulf oil and gas industry employs 107,000 people. By comparison, the area's tourism industry, which was devastated by the spill, employs 524,000.