Abstract: At about 3:30 p.m. on October 10, 1986, Employee #1, a self contractor, arrived to inspect the sawmill's fire protection sprinkler system backflow valve located in an underground vault 8 ft deep by 6 ft wide by 12 ft long with a 31 in. manhole opening. At about 4:20 p.m. a truck driver saw Employee #1 laying in 14 in. of water at the bottom of the vault and reported it to the mill office. Meanwhile, Employee #2 attempted to rescue Employee #1 and lost consciousness. Employee # 3 attempted to rescue Employee #2 and became disoriented. Employees # 4 through 7, the policeman and firemen paramedics, attempted rescue without proper equipment and required treatment for an oxygen deficient atmosphere with high levels of carbon dioxide after climbing out of the vault unaided. Firemen using self contained air respirators removed Employees # 1 through 3 from the vault, but were unable to revive Employees #1 and 2, who were pronounced dead at the hospital. Employees # 3 through 7 were hospitalized for asphyxia.