On March 20, the Department of the Interior released the final Deepwater Horizon autopsy report, Forensic Examination of Deepwater Horizon Blowout Preventor. You can view the entire document here, in this very large .pdf file. This report is full of great photos and illustrations, so it's worth the time to download it and look. All illustrations in this section are taken from the report linked here.

To save time, they replaced the drilling mud meant to supply backpressure in the pipes with seawater -- and it turned out the sea water couldn't hold back the oil. They attempted to deploy a blowout preventor, which failed miserably.

A network of shearing rams is normally in place, ready to cut and crimp the pipe so that the oil flow stops. At Deepwater Horizon, though, the pipe wasn't centered correctly with respect to the shearing blade. When they deployed the shearing ram, the pipe didn't get properly cut or sealed. Another shearing ram that would have been in place was not deployable, although it wouldn't have made a difference: it would have suffered the same failure as the first, because the pipe was off-center there, as well.

This created the leak that spilled for months.

