The Deepwater Horizon oil spill (also referred to as the BP oil spill, the BP oil disaster, the Gulf of Mexico oil spill, and the Macondo blowout) is an oil spill in the Gulf of Mexico which flowed unabated for three months in 2010, and may be continuing to seep. It is the largest accidental marine oil spill in the history of the petroleum industry. The spill stemmed from a sea-floor oil gusher that resulted from the 20 April 2010 explosion of Deepwater Horizon, which drilled on the BP-operated Macondo Prospect. The explosion killed 11 men working on the platform and injured 17 others. On 15 July 2010, the gushing wellhead was capped, after it had released about 4.9 million barrels (780,000 m3) of crude oil. An estimated 53,000 barrels per day (8,400 m3/d) escaped from the well just before it was capped. It is believed that the daily flow rate diminished over time, starting at about 62,000 barrels per day (9,900 m3/d) and decreasing as the reservoir of hydrocarbons feeding the gusher was gradually depleted. On 19 September 2010, the relief well process was successfully completed, and the federal government declared the well "effectively dead". In August 2011, oil and oil sheen covering several square miles of water were reported surfacing not far from BP’s Macondo well. Scientific analysis confirmed the oil is a chemical match for Macondo 252. The Coast Guard said the oil was too dispersed to recover. In March 2012, a "persistent oil seep" near the Macondo 252 well was reported.
The spill caused extensive damage to marine and wildlife habitats and to the Gulf's fishing and tourism industries. Skimmer ships, floating containment booms, anchored barriers, sand-filled barricades along shorelines, and dispersants were used in an attempt to protect hundreds of miles of beaches, wetlands, and estuaries from the spreading oil. Scientists also reported immense underwater plumes of dissolved oil not visible at the surface as well as an 80-square-mile (210 km²) "kill zone" surrounding the blown well. In late November 2010, 4,200 square miles (11,000 km²) of the Gulf were re-closed to shrimping after tar balls were found in shrimpers' nets. The amount of Louisiana shoreline affected by oil grew from 287 miles (462 km) in July to 320 miles (510 km) in late November 2010. In January 2011, an oil spill commissioner reported that tar balls continue to wash up, oil sheen trails are seen in the wake of fishing boats, wetlands marsh grass remains fouled and dying, and crude oil lies offshore in deep water and in fine silts and sands onshore. A research team found oil on the bottom of the seafloor in late February 2011 that did not seem to be degrading. On 26 May 2011, the Louisiana Department of Environmental Quality extended the state of emergency related to the oil spill. By 9 July 2011, roughly 491 miles (790 kilometers) of coastline in Louisiana, Mississippi, Alabama and Florida remained contaminated by BP oil, according to a NOAA spokesperson. In October 2011, a NOAA report stated that dolphins and whales continue to die at twice the normal rate. In April 2012, scientists reported finding alarming numbers of mutated crab, shrimp and fish they believe to be the result of chemicals released during the oil spill. Tar balls continue to wash up along the Gulf coast two years after the spill began.  In April 2012, oil was found dotting 200 miles of Louisiana's coast.
In January 2011 the White House oil spill commission released its final report on the causes of the oil spill. They blamed BP and its partners for making a series of cost-cutting decisions and the lack of a system to ensure well safety. They also concluded that the spill was not an isolated incident caused by "rogue industry or government officials", but that "The root causes are systemic and, absent significant reform in both industry practices and government policies, might well recur". After its own internal probe, BP admitted that it made mistakes which led to the Gulf of Mexico oil spill. In June 2010 BP set up a $20 billion fund to compensate victims of the oil spill. To July 2011, the fund has paid $4.7 billion to 198,475 claimants. In all, the fund has nearly 1 million claims and continues to receive thousands of claims each week.
In September 2011, the U.S. government published its final investigative report on the accident. In essence, that report states that the main cause was the defective cement job, and put most of the fault for the oil spill with BP, also faulting Deepwater Horizon operator Transocean and contractor Halliburton. Investigations continue, with U.S. Attorney General Eric Holder stating on April 24, 2012, "The Deepwater Horizon Task Force is continuing its investigation into the explosion and will hold accountable those who violated the law in connection with the largest environmental disaster in US history". The first arrest related to the spill was in April 2012; an engineer was charged with obstruction of justice for allegedly deleting 300 text messages showing BP knew the flow rate was three times higher than initial claims by the company, and knew that Top Kill was unlikely to succeed, but claimed otherwise.
At approximately 9:45 pm CDT, on 20 April 2010, high-pressure methane gas from the well expanded into the drilling riser and was released onto the drilling rig, where it ignited and exploded, engulfing the drilling rig. Most of the workers escaped the rig by lifeboat and were subsequently evacuated by boat or airlifted by helicopter for medical treatment; however, eleven workers were never found despite a three-day Coast Guard search operation, and are believed to have died in the explosion. Efforts by multiple ships to douse the flames were unsuccessful. After burning for approximately 36 hours, the Deepwater Horizon sank on the morning of 22 April 2010.