THE NATURE OF BACTERIAL GASES The near-surface underground is a diverse habitat for many species of bacteria. Among the mass of rock, soil, and water underground, bacteria may seem imma­ terial, but over time they can alter their environment in ways that affect under­ ground construction. Bacteria underground have no direct access to the energy of the sun; most acquire chemical energy by decomposing organic compounds incorporated in sediments. Among the waste products of bacterial decomposition are potentially hazardous amounts of methane, hydrogen sulfide, and carbon di­ oxide.