What exactly happened on the Deepwater Horizon on April 20, the day that the doomed oil rig that went up in flames and dumped millions of gallons of oil into the Gulf? The Wall Street Journal has finished its three-part investigation and finds that on that final morning a crucial change was put in motion. Workers found out that a safety test was needed. BP knew that more thick fluid, known as mud, needed to be removed from the well than usual. But the test confused the workers on board the rig. Managers disappeared when they might have been needed most. Even though the oil industry has top-flight technology, its workers rely on instinct. And the Deepwater Horizon hands didn't know what their instincts were telling them, the Journal concludes.