Here's why:

* Solubility: Limestone is primarily composed of calcium carbonate, which is soluble in weakly acidic water.

* Carbonic Acid: Rainwater absorbs carbon dioxide from the atmosphere, forming a weak acid called carbonic acid. This acid dissolves the limestone, creating cavities and eventually caverns.

* Other Factors: While limestone is the most common, other carbonate rocks like dolostone can also form caverns. The process is similar, as these rocks also contain minerals that are susceptible to acidic dissolution.

Let me know if you'd like to learn more about the formation of caverns!