Answer: Groundwater that forms caves because it is slightly acidic.
Step-by-step explanation:
The water under the ground present inside the cracks and spaces in sand, soil and rocks is called ground water.
When ground water moves through the soil it results in the formation of acid. Further, this acid helps minerals to get dissolve. These minerals get deposited on other places as well.
This process of deposition and erosion helps in the formation of caves.
Thus, it can be concluded that groundwater that forms caves because it is slightly acidic.