To understand how a geyser works we must understand that the temperature at which water boils rises with increasing pressure. At sea level (“one atmosphere pressure” or 760 millimeters of mercury) water boils at 100 degrees Celsius (212° Fahrenheit). In Denver, Colorado, USA, also known as the “Mile High City,” atmospheric pressure is less than at sea level and water boils at about 95°Celsius (203°F). Just the opposite happens when water is at greater pressure in the earth’s crust. High pressures increase the temperature required to boil water deep underground.