Abstract: On September 19, 2006, three employees were working on a sewer system that was newly installed and not yet in use. A section of the line had been plugged and tested for leakage. Employee #1 entered the sewer vault, which was approximately 15 to 20 feet deep, to remove a plug. Employee #1 collapsed into approximately 6 inches of unidentified liquid at the bottom of the sewer vault. Employee #2 entered the sewer vault to assist Employee #1. Employee #2 also collapsed at the bottom of the sewer vault. Employee #3 attempted to provide assistance to Employees #1 and #2. Employee #3 began to feel ill about halfway down and then decided to emerge from the sewer vault. Prince George's County, Maryland, Fire/EMS Department responded to the scene. Co-workers of the employees attached a hose approximately 19 feet long to an air compressor and used it to blow air into the sewer vault. Employee #2 regained consciousness and was able to assist in rescuing Employee #1 and himself from the sewer vault. All three employees were transported to area hospitals. Employee #1 later died at the hospital. Employees #2 and #3 were treated, hospitalized, and released in the following days.