Abstract: At approximately 7:40 on February 13, 1997, Employee #1 was working from a 30 ft high tubular welded frame scaffold, preparing to weld a 20 in. diameter by 1/4 in. thick metal hatch cover into place. He was at the west end of a rotary railroad car dumper that was being installed at a landfill under construction. When the employee struck an arc to begin welding, there was an explosion that propelled him off the scaffold and into the air. As he fell, he struck the top of an adjacent 15 ft high concrete wall and then landed on the concrete floor. He sustained massive facial and head injuries and was killed. The previous day, a mixture of metal shot and concrete had been poured into each end of the dumper for ballast. The five hatch covers at the east end of the dumper had been welded in place as the concrete was poured. Four of the five hatch covers at the west end of the dumper had also been welded in place as the concrete was poured. The fifth one, near the top, had not been welded but had been set in place in preparation for welding the next morning. Apparently, some type of explosive gas had been generated and built up inside the enclosed space overnight, causing the explosion. The identity of the gas is not known at this time.