Abstract: At about 4:20 p.m. on July 1, 2004, Employee #1 entered a manhole at a new installation, which was about 6 ft diameter by 10 ft deep, to ensure that the 10-in water pipeline valve was open prior to flushing and pressure testing the line. Employee #1 collapsed and fell unconscious against the valve. Employee #2 entered the manhole to attempt to rescue Employee #1; however, he had to exit the manhole before he ever reached the bottom. From the top of the manhole, the other employee attempted to assist Employee #2 to exit grabbing his arms, but Employee #2 slipped out of the grasp and fell unconscious to the bottom of the manhole. Fire Department personnel, wearing self contained breathing apparatus, removed Employees #1 and #2 from the manhole. Employee #1 was pronounced dead. Employee #2 was recovering in the hospital intensive care unit. An isotope of oxygen 17 (17 oxygen) was measured in the manhole by the Fire Department during the rescue.