Nearly 400 feet long, the Horizon had formidable and redundant defenses against even the worst blowout. It was equipped to divert surging oil and gas safely away from the rig. It had devices to quickly seal off a well blowout or to break free from it. It had systems to prevent gas from exploding and sophisticated alarms that would quickly warn the crew at the slightest trace of gas. The crew itself routinely practiced responding to alarms, fires and blowouts, and it was blessed with experienced leaders who clearly cared about safety.

On paper, experts and investigators agree, the Deepwater Horizon should have weathered this blowout.

This is the story of how and why it didn't.