At 10 p.m. on 3/19/18, an employee, 34, and coworker were performing preventative maintenance on a chlorine gas system used in a water filtration process. The employee and coworker were removing, and reinstalling, a flange with a black Teflon rubber gasket. They were working in a 48-foot long, 24-foot wide, and 6-foot tall crawl space under a chlorine cylinder storage building.