The Sullivan Mine No. 1 Shaft waste rock dump was built on a natural slope and covered by till. The outflow of O2–deficient gas through a leachate drainage pipe in an enclosure at the base of the dump resulted in four fatalities. A numerical model was developed to understand the mechanism controlling gas flow, which was found to be the relative buoyancy of the gas phase within the dump compared with atmospheric air. Changes in atmospheric air density are caused by atmospheric temperature variations, whereas dump gas-phase density is relatively constant due to a steady internal dump temperature. When the air temperature is lower than the internal dump temperature, atmospheric air density is higher than the dump gas density, inducing upward dump gas flow and air entry into the drainage pipe. Downward dump gas flow occurs and exits the drainage pipe when high atmospheric temperature leads to an air density lower than the dump gas density. A gas flow behavior similar to what was observed at the No. 1 Shaft dump could occur in other covered dumps.