Natural mineral water comes from springs deep in the ground, and contains large amounts of mineral salts or gases.
Some mineral water looks like ordinary tap water, but some has bubbles in it just like club soda. This is called carbonated water, for these bubbles are bubbles of carbon dioxide gas.
These bubbles form when limestone is present in the earth where mineral water springs originate. What happens is that the water reacts with the limestone and forms carbon dioxide gas. But underground pressure keeps the gas from escaping, and it remains in the water.
However, when the water springs from the ground, or is poured from a sealed bottle, carbon dioxide bubbles rise to the surface of the water and escape into the air.