The Boeing 737 is one of the most popular and profitable planes in the world and the backbone of the aviation industry. But in the 1990s, two deadly crashes and another near miss puzzled investigators, which sent them scrambling for answers. Their hunt led to the longest investigation the NTSB has ever conducted, with the fate of the airline industry hanging in the balance. Witness these mysterious and tragic accidents, and see how they led to a worldwide recall on 737s as well as sweeping changes in training protocols.