‘Deepwater Horizon,’ an Explainer: How a Faulty Plug Caused a Deadly Environmental Disaster On April 20, 2010, the Deepwater Horizon oil rig exploded, killing 11 people and leaking millions of barrels of oil into the Gulf of Mexico. The first film about the incident, Lionsgate/Summit’s “Deepwater Horizon,” starring Mark Wahlberg, hits theaters this weekend. While there’s always a certain amount of Hollywood burnishing on any film based on a true story, it’s helpful to know what really happened — and how a piece of cement played such a crucial role in the tragedy. According to the U.S.