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.Geysers are comprised of an intricate series of fractures and cracks. Narrow constrictions within the network of fractures act as pipes. When the fracture system is full of water, the pressure at the bottom of the geyser system is at a greater pressure, due to the weight of the overlying water column, than the water at the top of the geyser near the opening. The greater pressure at the bottom of the geyser keeps the deep water from boiling even though temperatures may reach 150°C (302°F). If the pressure of the overlying water column is reduced in some way, the effect is an immediate pressure drop throughout the water column in the geyser. This can initiate instantaneous high-temperature violent boiling at the bottom of the geyser that forces the water explosively out of the throat and mouth of the geyser. One mechanism that may start the process is for the water at the top to boil to the point where water (and pressure) is lost out of the geyser system allowing progressively deeper water to boil. The zone of boiling continues downward rapidly and increases in intensity forcing the water column out of the geyser.