Mexico City - Mexico's capital is sinking at a rate of up to 45 cm a year as the water-table is drained to supply the megapolis of 20 million people, according to a report just released. The capital has sunk 10 metres over the course of the century. "If the current rate of drainage continues, not only would it affect the quality and quantity of the underground water, it would also cause social and ecocomic problems for the population," the report said.