Three Thousand Years Underground: The Madison Aquifer

The Madison Aquifer is one of the largest confined groundwater systems in North America, spanning five states and three Canadian provinces beneath the northern Great Plains and Rocky Mountain Front. It is not a lake or an underground river. It is a vast formation of porous limestone saturated with water under pressure, moving imperceptibly through the rock.

Water enters where the limestone is exposed at the surface, in mountain ranges such as the Little Belts of Montana, the Bighorns of Wyoming, and the Black Hills, seeps in slowly, and eventually escapes at points like Giant Springs here at the edge of the Missouri. Using modern dating techniques, scientists estimate that the water emerging at Giant Springs has been in transit for approximately three thousand years, moving through bedrock since roughly the time of ancient Greece. The springs discharge at a steady fifty-four degrees Fahrenheit every single day of the year, regardless of what the Montana sky is doing above.