Water was the key resource that first attracted Mexico City's ancient residents, but may be a problem for the almost thirty million residents today. The interconnected lakes and deep aquifers that lie beneath the capital are drying up, and Mexico City is sinking. The water supply, treatment, distribution and waste disposal are all in jeopardy. And unless swift action is taken, the taps in the world's most populous city may soon run dry. Bob Carty reports.