Caused in the aftermath of a blowout and explosion on the Deepwater Horizon oil platform, the United States federal government estimated the total discharge at 4.9 million barrels (210,000,000 US gal; 780,000 m3).

[13] Due to the months-long spill, along with adverse effects from the response and cleanup activities, extensive damage to marine and wildlife habitats and fishing and tourism industries was reported.

The United States Government report, published in September 2011, pointed to defective cement on the well, faulting mostly BP, but also rig operator Transocean and contractor Halliburton.

[31] Built by South Korean company Hyundai Heavy Industries[32] and owned by Transocean, the rig operated under the Marshallese flag of convenience, and was chartered to BP from March 2008 to September 2013.

[35] At approximately 7:45 pm CDT, on 20 April 2010, high-pressure methane gas from the well expanded into the marine riser and rose into the drilling rig, where it ignited and exploded, engulfing the platform.

[115] Containment booms stretching over 4,200,000 ft (1,300 km) were deployed, either to corral the oil or as barriers to protect marshes, mangroves, shrimp/crab/oyster ranches or other ecologically sensitive areas.

[130][medical citation needed] Environmental scientists expressed concerns that the dispersants add to the toxicity of a spill, increasing the threat to sea turtles and bluefin tuna.

"[18] Another peer-reviewed study, released in March 2014 and conducted by 17 scientists from the United States and Australia and published in the Proceedings of the National Academy of Sciences, found that tuna and amberjack that were exposed to oil from the spill developed deformities of the heart and other organs that would be expected to be fatal or at least life-shortening.

[19] The oil dispersant Corexit, previously only used as a surface application, was released underwater in unprecedented amounts, with the intent of making it more easily biodegraded by naturally occurring microbes.

[202] According to J. Christopher Haney, Harold Geiger, and Jeffrey Short, three researchers with extensive experience in environmental monitoring and post-spill mortality assessments, over one million coastal birds died as a direct result of the Deepwater Horizon spill.

[211] [dead link‍] A peer-reviewed NOAA/BP study disclosed that nearly half the bottlenose dolphins tested in mid-2011 in Barataria Bay, a heavily oiled area, were in "guarded or worse" condition, "including 17 percent that were not expected to survive".

[215] NWF senior scientist Doug Inkley notes that the marine death rates are unprecedented and occurring high in the food chain, strongly suggesting there is "something amiss with the Gulf ecosystem".

[216] In December 2013, the journal Environmental Science & Technology published a study finding that of 32 dolphins briefly captured from 24-km stretch near southeastern Louisiana, half were seriously ill or dying.

[248] A 2012 survey of the health effects of the spill on cleanup workers reported "eye, nose and throat irritation; respiratory problems; blood in urine, vomit and rectal bleeding; seizures; nausea and violent vomiting episodes that last for hours; skin irritation, burning and lesions; short-term memory loss and confusion; liver and kidney damage; central nervous system effects and nervous system damage; hypertension; and miscarriages".

Dr. James Diaz, writing for the American Journal of Disaster Medicine, said these ailments appearing in the Gulf reflected those reported after previous oil spills, like the Exxon Valdez.

One study projects that the overall impact of lost or degraded commercial, recreational, and mariculture fisheries in the Gulf could be $8.7 billion by 2020, with a potential loss of 22,000 jobs over the same time frame.

[266][267] On 30 April 2010, President Barack Obama ordered the federal government to hold the issuing of new offshore drilling leases and authorized the investigation of 29 oil rigs in the Gulf in an effort to determine the cause of the disaster.

[272] On 28 April 2010, the National Energy Board of Canada, which regulates offshore drilling in the Canadian Arctic and along the British Columbia Coast, issued a letter to oil companies asking them to explain their argument against safety rules which require same-season relief wells.

[278] On 30 April, President Obama dispatched the Secretaries of the Department of Interior and Homeland Security, as well as the EPA Administrator and NOAA to the Gulf Coast to assess the disaster.

In May 2013, he was honored as a "distinguished leader" by the University of Birmingham, but his award ceremony was stopped on multiple occasions by jeers and walk-outs and the focus of a protest from People & Planet members.

[308] The National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling was established on 22 May to "consider the root causes of the disaster and offer options on safety and environmental precautions.

[318] The report stated that "whether purposeful or not, many of the decisions that BP, Halliburton, and Transocean made that increased the risk of the Macondo blowout clearly saved those companies significant time (and money).

"[332] By 26 May 2010, over 130 lawsuits relating to the spill had been filed[333] against one or more of BP, Transocean, Cameron International Corporation, and Halliburton Energy Services,[334] although it was considered likely by observers that these would be combined into one court as a multidistrict litigation.

[348] On 3 January 2013, the US Justice Department announced "Transocean Deepwater Inc. has agreed to plead guilty to violating the Clean Water Act and to pay a total of $1.4 billion in civil and criminal fines and penalties".

[59] On 25 July 2013, Halliburton pleaded guilty to destruction of critical evidence after the oil spill and said it would pay the maximum allowable fine of $200,000 and will be subject to three years of probation.

[350] In January 2014, a panel of the U.S. Fifth Circuit Court of Appeals rejected an effort by BP to curb payment of what it described as "fictitious" and "absurd" claims to a settlement fund for businesses and persons affected by the oil spill.

[351][352] The Supreme Court of the United States later refused to hear BP's appeal after victims and claimants, along with numerous Gulf coast area chambers of commerce, objected to the oil major's efforts to renege on the Settlement Agreement.

A finding of gross negligence would result in a four-fold increase in the fines BP would have to pay for violating the federal Clean Water Act, and leave the company liable for punitive damages for private claims.

[358] According to the plaintiffs' lawyers the major cause of an explosion was the mishandling of a rig safety test, while inadequate training of the staff, poor maintenance of the equipment and substandard cement were also mentioned as things leading to the disaster.

Site managers Donald Vidrine and Robert Kaluza were charged with manslaughter for acting negligently in their supervision of key safety tests performed on the rig prior to the explosion, and failure to alert onshore engineers of problems in the drilling operation.