Abstract: Apparently Employee #1 was atop an oil field frac tank that contained drilling mud, and which had sat on-site for 60 days. He was using a water hose to dilute the mud for easier pump removal into a tank truck for disposal. He was overcome as he either leaned into or over the tank hatch opening to inject the water into the mud. Employees #2 and #3 then entered the tank, either one at a time or together, to rescue the first employee. One employee died from methane gas inhalation and two apparently were overcome by the gas and fell into the drilling mud and drowned. This frac tank differed from the others they had already emptied in that, because it did not have a front hatch opening, it had to be diluted from the top hatch. No respirators were on-site because the employees were not entering the tanks to do the job.