One fatality caused by CO migrationg though soil after nearby use of explosives.
Explosive blasts (nitroglyerine-based) were used to break up rock layers 6 feet below the surface before excavating pipeline trenches and manhole pits. A worker entered a 12 feet deep manhole 40 - 60 feet from the blast area 45 minutes after the blast. The worker collapsed in the manhole. Two co-workers went to rescue him. One of the rescuers collapsed on the ground after retrieving the first casualty. The second rescuer died in the manhole. Carbon monoxide levels measured in the base of the manhole two days after the incident were 1905 ppm. Levels of 1200ppm are considered immediately dangerous to life and health.
Source: Applied Occupation and Environmental Hygiene, Vol 17(3) 152-153, 2002