Mexico City, one of the world's largest cities, is rapidly subsiding at a rate of nearly 10 inches per year, causing it to risk displacement of millions of citizens. Satellite photos by NASA have revealed that the city, which was originally built on an ancient lake bed, is eroding at a rapid rate due to groundwater pumping and urban development. Researchers used a powerful NASA satellite to monitor the subsidence rate every 12 days, allowing scientists to see what was happening underneath in real time. Some parts of the city are experiencing an annual subsidence of 0.78 inches per month, making it one of Mexico's fastest-sinking cities. The shrinking aquifer could also result in a major water shortage.