Answer:
Because of the lime rock that is present in the caverns.
Step-by-step explanation:
In the evolution of Earth, the different kind of soils that grows in an area is product of the leftovers of other living things in ancient times.
In the case of the Santee Carverns, the floor of these caverns are made of limestone, that is conform with a high concentration of calcium and magnesium in its composition, that mean, that in ancient times, both minerals were from coral reefs. On coral reefs, there is always a high concentration of these minerals, that is reason why coral reefs can maintain the carbon dioxide in water sea.
Hope this info is useful.