“The Geysers” spanning an area of around 30 square miles (78 km2) in Sonoma, Lake, and Mendocino counties is the world's largest geothermal field, containing a complex of 18 geothermal power plants and drawing steam from more than 350 wells.
Geothermal gases come from volcanic activity deep within the Earth. In affected areas, gases sometimes rise to the Earth's surface through small cracks and other openings. The small openings in the ground where gases escape are called "fumaroles" or "vents". Hot springs result from the deep venting of hot water and may also contain gases.
Geothermal gases can be dangerous when found in high concentrations, but are rapidly diluted to harmless levels when they are exposed to fresh open air. Gases can also travel through soil and into indoor air through cracks in foundations and utilities. Houses with cement foundations ("slab-on-grade") have the greatest risk for gases moving from underground into indoor air. These gases can make people sick if they accumulate in enclosed spaces, buildings or low-lying areas such as a basement or crawl space. At high levels they can cause suffocations or sometimes explosions. Children and pets are most sensitive to the effects of geothermal gases because they are closer to the ground.