Subsidence is the motion of the Earth's surface as it shifts downward.
Land subsidence occurs when large amounts of groundwater have been excessively withdrawn from an aquifer (an aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials). The clay layers within the aquifer compact and settle, resulting in lowering the ground surface in the area from which the groundwater is being pumped.
Over time, as more water is removed from the area, the ground drops. Once the water has been removed from the sediment, it cannot be replaced.