Giant Springs (Wikipedia)
Giant Springs is a large first magnitude spring located near Great Falls, Montana and is the central feature of Giant Springs State Park. Its water has a constant temperature of 54 °F (12 °C) and originates from snowmelt in the Little Belt Mountains, 60 miles (97 km) away. According to chlorofluorocarbon dating, the water takes 26 years to travel underground before returning to the surface at the springs.
Giant Springs is formed by an opening in a part of the Madison aquifer, a vast aquifer underlying 5 U.S. States and 3 Canadian Provinces. The conduit between the mountains and the spring is the geological stratum found in parts of the northwest United States called the Madison Limestone. Although some of the underground water from the Little Belt Mountains escapes to form Giant Springs, some stays underground and continues flowing, joining sources from losing streams in the Black Hills, Big Horn Mountains and other areas. The aquifer eventually surfaces in Canada. Giant Springs has an average discharge of 242 cubic feet (6.9 m3) of water per second.