Overexploitation has required wells to become deeper. They range up to a depth of 400 metres, where a depth of 70 metres was sufficient for older wells.6 Due to the soil composition, extraction from deeper wells tends to be less efficient. World Bank, Washington, DC, available at: www.agua.uiiam.mx/sacinex/assets/docs/AcuaUitoana ValleMexic o.pdf. Reduction in water in the clay soil that supports a large part of the city leads to densification of the soil and subsidence.