Deepwater Horizon was an ultra-deepwater, dynamically positioned, semi-submersible offshore drilling rig owned by Transocean and operated by BP. On 20 April 2010, while drilling at the Macondo Prospect, a blowout caused an explosion on the rig that killed 11 crewmen and ignited a fireball visible from 40 miles away. The fire was inextinguishable two days later, on 22 April, the Horizon sank, leaving the well gushing at the seabed and turning into the largest marine oil spill in history.