Despite its terrible environmental rap, Mexico City remains one of the greenest cities in the world, with more than half the city’s acreage designated as open space and fully 25 percent blanketed with forest. Unfortunately, all that is being threatened by the city’s uncontrolled urban sprawl, most of it in the form of creeping shantytowns forged of poverty and desperation on the city’s edges. As many as 260,000 people live in an estimated 65,000 illegal homes built inside conservation areas on the city’s south side. As conservation areas disappear, so do the trees that mitigate air pollution and erosion, and the aquifer that supplies 70 percent of the city’s water.