Abstract: Workers were pumping drilling mud from frac tanks to a pumper truck, which involves hooking the pump truck to the frac tank and pumping mud down to a level below the frac tank's front hatch. Removing the front hatch allows spraying water to dilute the mud for easier removal. Employees #1 and #2 were pumping out a frac tank that was different from the other tanks as it did not have a front hatch. Apparently, Employee #3 had arrived at the well site and hooked up his pump truck to this frac tank and Employee #2, without a safety belt and lanyard, climbed on top of the 16 ft tank and was spraying water into the tank through the top hatch. Employee #2 was overcome by the methane gas and fell into the tank. Employees #1 and #3 attempted to rescue him from inside the tank but were overcome by methane gas and drowned in the drilling mud.