What caused the explosion on the oil rig ?

The deadly blast on board the Deepwater Horizon oil rig in the Gulf of Mexico was triggerd by a bubble of methane gas. A report into the explosion said gas escaped from the oil well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding.It caused the 'Deepwater Horizon' to burn and sink, and started a massive off shore oil spill in the Gulf of Mexico, which is now considered the largest U.S history and environmental disaster. The explosion killed 11 people and injured 17 others, 98 others survived without serious physical injury.The Deepwater Horizon was a floating semi-submersible Drilling Unit — a fifth-generation, ultra-deepwater, dynamically positioned, column-stabilized. The platform was 396 feet (121 m) long and 256 feet (78 m) wide and could operate in waters up to 8,000 feet (2,400 m) deep, to a maximum drill depth of 30,000 feet (9,100 m).