More than 150 million gallons of water flow from Giant Springs every day and cascade into the Missouri River. The springs form where cracks in rocks above the Madison Limestone allow water to leak upward to the land surface, like water under pressure escaping through an opening in a hose. Tests have found the spring water to be very pure, although it is not advisable to drink directly from the springs because birds and other animals live in and use the water. More than 200 years after William Clark called it the largest fountain or spring he had ever seen, saying the water boiled up from under the rocks near the river's edge, fell immediately into the river, and kept its immensely clear bluish color for a mile, his description remains true.