Detection of gases, associated with all mineral deposits, can lead to the discovery of deposits beneath the ground surface. Carbon dioxide, hydrogen sulfide, and other gases are common constituents of the hydrothermal solutions that transport ore minerals. Other gases are produced when those minerals react with ground water or with microorganisms. Hydrocarbon liquids and gases such as methane and ethane are produced when petroleum and natural gas deposits are formed. Hydrocarbon gases are also found around metallic ore deposits. All gases have a natural tendency to migrate from areas of high pressure to areas of low pressure, that is, from depth to the surface. They migrate slowly by diffusion through rock, or rapidly through permeable zones such as faults, fractures, or breccia zones. The gases are sampled at or near the surface, and are analyzed to measure gas species and their concentration. When the results are plotted on maps, anomalous areas of high or low concentrations are readily seen and may indicate buried mineral deposits or faults.