1. Caves: When groundwater is slightly acidic, it can dissolve limestone over long periods, creating underground cavities called caves. These caves can be very large and complex, often containing formations like stalactites and stalagmites.

2. Sinkholes: These depressions in the Earth's surface form when underground rock, like limestone, is dissolved by acidic groundwater. The overlying soil or rock can then collapse into the empty space, creating a sinkhole. Sinkholes can range in size from small depressions to large craters.