Plot: Long before the Deepwater Horizon explosion in the Gulf, BP was widely viewed as a company that valued deal-making and savvy marketing over safety, a "serial environmental criminal" that left behind a long trail of problems -- deadly accidents, disastrous spills, countless safety violations -- which many now believe should have triggered action by federal regulators. Could the spill have been prevented? Through interviews with current and former employees and executives, government regulators, and safety experts, FRONTLINE correspondent Martin Smith joins with the investigative non-profit ProPublica to examine the trail that led to the disaster in the Gulf