Deepwater Horizon was the name of an oil rig that malfunctioned and exploded in the Gulf of Mexico on April 20, 2010, killing 11 crew members and creating a disastrous flow of oil into the ocean.
The Deepwater Horizon rig was owned by Transocean and was being leased by BP for deepwater drilling at one of BP's wells, the Macondo Prospect. BP's inability to stop the oil leak were deemed inadequate and irresponsible by many. (The leaking well has not been capped as of this writing, June 1, 2010, despite several tactics tried by BP.)
One thing making it difficult to stop the leak is the well's depth.