For months, scientists have worried that drought around Mexico City, which has over 22 million residues, would cause the city to run out of water. That day is approaching fast, and there is no alternative. Some groups of people in the city or some set of essential services will suffer within a few months or even weeks. The reservoirs of the Cutzamala water system, which provide about one-fifth of the city’s water, are essentially dry. The only available option is to attempt to take water from the aquifers below the city. There is not a large enough supply, and as...