Deepwater Horizon was an ultra-deepwater, semi-submersible offshore drilling rig owned by Transocean and operated by BP. On April 20, 2010, while drilling in the Gulf of Mexico at the Macondo Prospect, a blowout caused an explosion on the rig that killed 11 crew members and started a massive fire. The rig ultimately collapsed and sank two days later, resulting in the largest marine oil spill in history. This spill caused extensive environmental damage and coastal pollution, with an estimated 134 million gallons of oil released into the Gulf of Mexico. Deepwater Horizon was notable for drilling the deepest oil well in history prior to the disaster.