Carbon monoxide (CO) is an odorless, colorless gas that can cause illness and death by asphyxiation. Although the toxicity of CO is understood, occupational CO exposure can occur from unrecognized sources. In a recent incident, three cases of CO poisoning in a confined space, including one fatality, were caused by CO migrating through soil after nearby use of explosives. A municipal sewer project involved the installation of new pipes and manholes. Explosive blasts were used to break up rock layers 6 feet below the surface before excavating pipeline trenches and manhole pits. On the day of the fatality, a construction crew installed a 12-foot-deep manhole without incident. After the crew left the area, 265 pounds of nitroglycerin-based explosive in 20 boreholes, each 18 feet deep, were detonated 40-60 feet from the manhole. A worker who entered the manhole 45 minutes after the explosion collapsed within minutes, and two coworkers descended into the manhole to rescue him. One rescuer retrieved the unconscious worker before collapsing on the surface, and the other rescuer died in the manhole. All involved construction workers had elevated blood levels of carboxyhemoglobin indicating they had inhaled air containing high CO concentrations. An investigation determined that carbon monoxide released from the explosion had migrated through the soil into the manhole. CO concentrations in the bottom of the manhole 2 days after the incident were 1,905 parts per million (ppm), well above the immediately dangerous to life and health (IDLH) concentration of 1,200 ppm. Tests following ventilation of the manhole showed that high levels of CO reappeared as a result of continued diffusion from the surrounding soil. Subsequent monitoring of the manhole showed a decline in CO levels over the next 8 days. This incident illustrates that CO from subsurface detonations of explosives can migrate underground and accumulate in confined spaces. This report is apparently the first occupational fatality from this type of CO exposure, though nonfatal CO poisonings have been reported in residential basements following nearby use of subsurface explosives. This incident also involved a "chain-reaction" death, a well-known danger associated with confined space rescues. Chain-reaction deaths are so named because after the first victim is found in a confined space, a rescuer enters without proper precautions and is overcome, a subsequent rescuer enters and is likewise overcome, and so on. Chain-reaction rescuer fatalities have accounted for 36% of the deaths in confined spaces.