Here's why:

* Solubility: Limestone is primarily made up of calcium carbonate, which is soluble in slightly acidic water.

* Carbonic Acid: Rainwater absorbs carbon dioxide from the atmosphere, creating carbonic acid.

* Dissolving Process: As slightly acidic water seeps through cracks in limestone, it dissolves the calcium carbonate, creating larger openings and eventually caves.

While limestone is the most common rock type for cave formation, caves can also form in:

* Gypsum: This rock is also soluble in water.

* Salt: Salt caves form in areas with salt deposits.

* Lava Tubes: These are formed by flowing lava, and can be quite large.

* Glacial Caves: Formed by the melting of glaciers.

Let me know if you'd like to know more about any of these cave types!