Fumaroles of cold, dry CO₂ such as this one on a lava flow erupted in 1938 from Nyamuragira Volcano, Zaire, can be exceptionally dangerous because the CO₂ discharge is invisible. The density difference between CO₂ and air is so great that a sharp boundary can persist between them. Lowering the cloth torch (top) just a few centimeters into the high concentration of CO₂ gas causes the flame to go out (bottom).

Such pockets of lethal CO₂ gas concentrations are common along the East African Rift Zone, and are the source of legends regarding elephant graveyards. The Swahili word for such pockets of gas is mazuku.