Another nice question. Thanks reader Joanie! Carbonation doesn’t just happen in seltzer bottles or in bottling plants, it happens in nature too. Wherever water and CO2 have the opportunity to mingle, you’ll get both dissolved carbon dioxide and carbonic acid. Most commonly that opportunity comes when underground pockets of volcanically-produced carbon dioxide gas come in contact with underground springs. Provided there’s enough sustained pressure to keep the CO2 from escaping into cracks in the earth, naturally-fizzy water will sometimes bubble up to the surface.
Carbonation can happen in other ways too. If you ever took advanced high school and/or college-level chemistry, you may recall an experiment whereby your instructor dribbled sulfuric acid onto a piece of limestone. The result is a prodigious amount of CO2. Something very similar (if not that very thing) can happen in nature, whereby atmospheric water vapor can come into contact with CO2 to form carbonic acid. When that water falls to earth as weak acid rain, it can enter the water table, come into contact with limestone and bingo: a carbonated spring.
Here I should note that spring water doesn’t have to be carbonated to be classified as “mineral water”, it just has to have some sort of dissolved minerals in it. Most mineral water is in fact carbonated artificially after it’s collected from a spring…assuming it’s collected from a spring at all…but don’t get me started on the topic of bottled water fraud. We’ll be here all day.