Mexico City's water crisis – from source to sewer

Each drop of water that passes through the Mexican capital tells a heroic, tragic, unfinished story of urban growth and human development. Over the course of a week, the Guardian follows this complex, costly trail

When a tormenta sweeps in to Mexico City, the rain does not just fall, it insists. Gently at first with a mid-afternoon patter on windows and windscreens, then more urgently with an evening downpour that turns splashes into puddles, until finally - with a nighttime climax of thunder and lightning rolling down from the distant volcanos - the deluge gushes through gutters and gullies, transforming trickles in runnels into torrents in tunnels. The floods are a reminder of the natural order of things: water belongs here.

This geological, historical fact is a reason why the Aztecs built a city of floating gardens here 700 years ago that became known as "the Venice of the New World". The vast lakes that once filled the plain were, however, steadily drained by settlers. In the 16th century, Spanish conquistadores rapidly accelerated the process, and modern engineers have almost finished the task, replacing the lacustrine marshes with a grey sea of concrete, tarmac and steel that, in the central city alone, is now home to almost nine million residents.