On April 20 at 10:43 a.m., a young BP PLC engineer sent a 173-word email to colleagues aboard the Deepwater Horizon drilling rig. The email spelled out a recent change to a key safety test that sparked confusion and debate aboard the rig.

Less than 12 hours later, the rig was engulfed in flames so hot they melted steel. Eleven workers were dead.

The worst offshore oil spill in U.S. history had begun.

The Deepwater Horizon blowout has become one of the most scrutinized maritime disasters ever. Congressional investigators and outside experts have identified a series of decisions in the weeks beforehand that made the blowout more likely.

But a central uncertainty remained: Why didn't the crew recognize the warning signs in the final hours and bring the well under control while there was still time to prevent a lethal eruption?

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The Wall Street Journal has reviewed BP internal documents along with hours of public testimony before a joint Coast Guard and Interior Department panel. The Journal also interviewed dozens of witnesses to the disaster. What emerges is a startling picture of the last day of the Deepwater Horizon—a day filled with disruption and disagreement.

Many workers on the rig didn't find out until the morning of April 20 about the change in a pressure test that would help determine the well's safety. BP wanted to remove an unusually large amount of the thick drilling fluid called mud from the well and then run the test. It was unorthodox and left crew members confused.

The oil industry employs extraordinary cutting-edge technologies. BP uses some of the world's fastest computers to locate oil reservoirs. Underwater robots tinker with wells beneath a mile of water.

But the truth about the modern oil industry is that it often relies on the judgment and instinct of men—and they are overwhelmingly men.

Continue reading at The Wall Street Journal