Abstract: Two employees of an electrical contractor were working in a 7.9-meter-deep sump manhole at a water desalination facility site under construction. An employee of a general contractor found the employees unconscious at the bottom of the manhole. An outside rescue service from a local fire department responded and found the atmosphere in the manhole to contain 8 percent oxygen at the bottom of the sump. The two employees died of hypoxic asphyxia. Postaccident evaluations found oxygen levels as low as 2 percent and elevated levels of nitrogen and carbon dioxide. The sump was found to be in contact with warm, moist soil through a series of interconnected perforated pipes designed to drain excess groundwater. It was suspected that biological activity in the surrounding soil consumed the available oxygen and generated excess levels of nitrogen and carbon dioxide.