The Perdido Oil rig, also in the Gulf of Mexico, will drill from 7200 feet below the surface to reach oil another 6000 feet below that.

The Deepwater Horizon drilled at a point under 5000 feet of water. And tapped into oil and gas with enough pressure to drive the drill string back up 30000 feet of pipe to the surpface and set fire to the drilling platform and break multiple holes in the pipe. The wellhead is under 150 times the pressure on the surface, in an environment cold enought to create mathane hydrates, a mixture of methane help within a lattice of water molecules.

The Blowout Preventer is a series of hydraulic rams which control the pressure at the wellhead, including one pair which are supposed to sever the well and shut it off completely, but the power lines to the BOP has been severed. Until the well is shut in and the well stack can be brought up, we don't really know what failed.