Why a spring can fizz while its water stays cool
Water underground is under pressure from the rock and water above it. That pressure helps keep gases dissolved in the water, much as pressure keeps carbon dioxide in a sealed soda. As groundwater moves toward the surface, the pressure eases. Some dissolved gas can then separate from the water and form bubbles.
The bubbles rise, expand, and burst at the surface, making a spring look active or even turbulent. Their presence does not mean the water is turning to vapor. They are gas that was mixed into the water, not necessarily steam made by heating it.
Boiling is a different process
Boiling happens when a liquid becomes vapor throughout its bulk, producing bubbles of the liquid’s own vapor. It occurs when the liquid’s vapor pressure reaches the surrounding pressure; at typical surface conditions, water must be heated close to its familiar boiling point. A spring can therefore bubble vigorously at a much lower temperature if gas is escaping from it.
That distinction matters when judging a spring by sight alone. A roiling surface may look hot, but appearance cannot tell you whether the water is boiling—or even whether it is warm. The motion comes from gas leaving the water; the temperature depends on a separate source of heat. A quiet-looking spring can be hot, while a lively one can remain lukewarm.
