A spring is an outlet for underground water. This means that water from underground aquifers or groundwater systems flows up to the surface through a spring. When water from rain, snow or other forms of precipitation seeps into the ground, it collects in underground rock formations or porous soil layers. These underground water reserves are also known as aquifers. When the pressure from the water in the aquifer becomes greater than the pressure holding the water down, the water flows up to the surface through a spring. Springs can be found in various geological settings, but they are most commonly associated with underground water systems. Therefore, a spring is not an outlet for rivers, streams in Karst regions, rain water in limestone regions, or volcanic lakes.