What Happened On The Deepwater Horizon? - Illustration by Emmett Mayer & Dan Shea

On April 20, 2010, the drilling rig Deepwater Horizon was two days away from temporarily capping the oil well that it had drilled and was in the process of handing off the pumping of the oil to a production platform or pipeline when oil and gas from the well came up through the pipe and caused an explosion that resulted in the deaths of 11 crew members, the sinking of the Deepwater Horizon and possibly the biggest environmental disaster in the history of continental United States.

Here's what happened:

When an oil well is being drilled, a pipe is inserted into the hole as the drill bit progresses. Drilling mud is pumped into the pipe and it flows down and around the bottom of the drill bit allowing debris to be carried back up the outside of the pipe. After the well has been drilled, cement is pumped around the outside of the pipe to seal and reinforce it. Once the cement has set, the drilling mud is removed from the pipe.

The disaster started as the drilling mud was being removed from the pipe. This occurred because the cement on the exterior of the pipe failed allowing seawater to enter the pipe. As drilling mud was being pumped from the pipe and seawater was leaking into it, the pressure inside the pipe dropped low enough that it was no longer sufficient to keep the oil and gas in the well. At about 10:00 p.m., the oil and gas mixture reached the surface and caused an explosion.

As the Deepwater Horizon was burning, operators tried unsuccessfully to activate the Blow Out Preventer, a device designed to plug the well.