Deepwater Horizon was an ultra-deepwater, dynamically positioned, semi-submersible offshore drilling rig[7] owned by Transocean and operated by the BP company.

On 20 April 2010, while drilling in the Gulf of Mexico at the Macondo Prospect, a blowout caused an explosion on the rig that killed 11 crewmen and ignited a fireball visible from 40 miles (64 km) away.

[8] The fire was inextinguishable and, two days later, on April 22, the Horizon collapsed, leaving the well gushing at the seabed and becoming the largest marine oil spill in history.

[9][10] Built in 2001 in South Korea by Hyundai Heavy Industries,[11] the rig was commissioned by R&B Falcon (a later asset of Transocean),[12] registered in Majuro, and leased to BP from 2001 until September 2013.

[15] Deepwater Horizon was a fifth-generation, RBS-8D design (i.e. model type), deepwater, dynamically positioned, column-stabilized,[2] semi-submersible mobile offshore drilling unit, designed to drill subsea wells for oil exploration and production using an 18.75 in (476 mm), 15,000 psi (100,000 kPa) blowout preventer, and a 21 in (530 mm) outside diameter marine riser.

[10] Transocean, through its Steinhausen, Switzerland[22] subsidiary[23] Triton Asset Leasing GmbH,[1][6] operated the rig under the Marshallese flag of convenience.

[40] In February 2010, Deepwater Horizon commenced drilling an exploratory well at the Macondo Prospect (Mississippi Canyon Block 252), about 41 miles (66 km) off the southeast coast of Louisiana, at a water depth of approximately 5,000 feet (1,500 m).

[43] Deepwater Horizon was still working on the Macondo site on 20 April 2010, when a violent explosion occurred resulting in destruction of the rig and the subsequent oil spill.

[50] The rig was scheduled to move to its next roles as semi-permanent production platforms, initially at the Tiber site followed by a return to the Kaskida field, an oil dome 50 miles off the coast of Louisiana.

[51] At 7:45 p.m. CDT on 20 April 2010, during the final phases of drilling the exploratory well off the gulf of Mexico, a geyser of seawater erupted from the marine riser onto the rig, shooting 70 metres (230 ft) into the air.

The gas component of the slushy material quickly transitioned into a fully gaseous state and then ignited into a series of explosions and then a firestorm.

[54] Ten workers were presumed killed by the initial explosion: An eleventh was crane operator Aaron Dale Burkeen, 37, Philadelphia, Mississippi, who died in the subsequent fire.

[61] Litigation, ultimate assessment of damage, and the scope of final insurance recovery were all unknown as of June 2010[update], with analysts reporting that the aftermath was of unprecedented scale and complexity compared to previous disasters which themselves took many years to resolve.

[62][63] A July 2010 analysis by the Financial Times of the aftermath cited legal sources as saying that "at some point the scale of the litigation becomes so large that it really is novel", that "the situation is likely to be complicated further because the variety of probable cases means it will be hard to aggregate them into so-called class actions" and that there was "no way to put this in historical context because we have never faced anything like this before".

[66] On 4 September 2014, U.S. District Judge Carl Barbier ruled BP was guilty of gross negligence and willful misconduct by the Clean Water Act (CWA).