On Sept. 13, 2018, at 4:04 p.m., an alarm sounded at a natural gas monitoring center in Columbus, Ohio. High-pressured natural gas had just been released into a low-pressure gas line in Massachusetts’ Merrimack Valley. Soon,buildings in Lawrence, Andover and North Andover would explode and catch fire. Thousands of people would be ordered to flee their homes and seek safety on the streets. WGBH Reporters were there to collect their stories and get answers to the questions on everyone’s mind: How did this happen? And, could it happen again?