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

Fact 2: Underground caves and caverns pose an exciting adventure to many.

Given the two facts above, answer the question "What can be formed by carbonic acid in groundwater seeping through rock and dissolving limestone?" with the following options: 
- water
 - swamp vegetation
 - Calcium carbonate
 - sunlight
 - contamination
 - loose soil
 - underground caves
 - arsenic


 

       underground caves  