If acavern is formed by carbon dioxide is in groundwater seeping through rock and dissolving limestone, then what forms caverns by seeping through rock and dissolving limestone?

Answer choices:
- pure oxygen
 - Something with a head, thorax, and abdomen
 - basic building blocks of life
 - carbon dioxide in groundwater
 - magma in groundwater
 - oxygen in groundwater
 - At the peak of a mountain
 - underground systems

 

    carbon dioxide in groundwater      