Abstract: On June 20, 2007, Employees #1 and #2 were working in an approximately 7 ft diameter water vault located about 16 ft underground. The vault contained a 12 in. water main and a 4 in. water main that was equipped with a water meter. The vault had been constructed approximately ten days earlier and had sat undisturbed until the day of the accident, when the employees were schedulued to conduct a pressure test of the system. Employee #1, the foreman, went down into the vault to read the meter. When he did not return, Employee #2, a laborer, looked down through the manhole cover and saw Employee #1 laying on the ground. Employee #2 called out to a coworker that Employee #1 was down and then entered the vault through the manhole and climb down the ladder. The coworker came over to the manhole and saw Employee #1 on the ground and Employee #2 hanging upside down, with his leg caught between the ladder rungs. Neither employee responded to the coworker's calls and he started down the manhole. The coworker also started down the manhole but noticed an overpowering musty odor and abruptly stopped and exited. The Fire Department and paramedics responded to the job site and retrieved Employees #1 and #2, both of whom had died. At the time of rescue the Fire Department's four gas meters measured the oxygen level in the vault at approximately 9.2 ppm. In its referral to OSHA, the Fire Department referenced two workers who succumbed to an IDLH atmosphere.