The Deepwater Horizon explosion has been studied by investigators and experts as much or more than any maritime disaster in history. But the key question remains: why didn't the crew recognize the warning signs when there was still time to prevent a catastrophe. After reviewing internal corporate documents and hours of public testimony and interviewing dozens of witnesses, the Wall Street Journal has emerged with "a startling picture" Deepwater Horizon's last day. Ben Casselman is part of the reporting team.