Fact 1: Acavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone.

Fact 2: Caverns Secrets are hidden in caves.

Given the two facts above, answer the question "what are formed by carbonic acid in groundwater seeping through rock and dissolving limestone?" with the following options: 
- aqueous solution
 - barrel-shaped
 - Calcium carbonate
 - Greenhouse gasses
 - hidden caves
 - a solute
 - swamp vegetation
 - a hydraulic system


 

     hidden caves    