When groundwater mixes with carbon dioxide from the air, it forms carbonic acid (H2CO3).

Here's how it happens:

* Carbon dioxide dissolves in water: Carbon dioxide (CO2) from the atmosphere dissolves into the groundwater.

* Reaction with water: The dissolved CO2 reacts with water (H2O) to form carbonic acid:

```

CO2 + H2O ⇌ H2CO3

```

This carbonic acid is a weak acid that can then react with minerals in the rocks and soil, leading to the dissolution of these minerals. This process is known as carbonation, and it plays a major role in the formation of caves and other karst features.

Here are some additional points:

* Carbonation is a key process in the rock cycle.

* The acidity of groundwater can be influenced by other factors besides CO2, such as pollution.

* The formation of carbonic acid is a reversible reaction, meaning that it can go in both directions.

Let me know if you have any more questions!