Abstract: Two employees entered a 16-foot-diameter drop shaft under construction on the Deep Tunnel Project. They were asphyxiated from a lack of oxygen, which had been displaced by carbon dioxide overnight. The vertical shaft, only 20 feet deep at the time, had been covered at the end of the previous workday, creating a confined space. Carbon dioxide from an undetermined source accumulated in the hole, displacing the oxygen. The two employees entered the shaft in a cage and immediately developed breathing difficulties. One of two (Employee #1) fell out of the cage as it was being pulled from the shaft. Four other employees went down in the cage to rescue him and were overcome as well. All six employees were sent to the hospital. Employee #1 died approximately 1 hour later. No tests of the air quality in the shaft were performed before entry.