INTRODUCTION Gases are common constituents of the underground environment, along with soils, rocks, and water. Natural gases present underground originate from the atmosphere, the mantle, bacterial decomposition of organic matter, thermal de­ composition of organic and inorganic compounds, and geochemical reactions. Gases are usually widely dispersed in low concentrations, but under some geo­ logic conditions are abundant. Gases can be a hazard to construction of under­ ground civil works such as tunnels, shafts, and underground chambers. Gases tend to be drawn into excavations by geostatic, hydrostatic, and barometric pressure reductions associated with the excavation. Gases from the ground may mix with air in the excavation to generate an explosive, toxic, or asphyxiating atmosphere.