Caverns
Caverns form when slightly acidic grounwater (usually w/ dissolved CO2) dissolves limestone or dolomite along fractures in the rock. This turns small cracks into large chambers. As more rock is dissolved away, groundwater has more room to flow, meaning that as caverns form, the water table tends to descend as well. This is why caves are not always filled with water even though groundwater movement is the primary driving force in their formation.