in the remote neighbourhood of san gregorio, one local initiative is tackling the greatest urban challenge in history. the more you look at the water situation in mexico city, the more shocking it is and the more you realize that we have to do something and we have to do it now. - enrique cofounded isla urbana. he has been setting up rainwater harvesting systems throughout the city for about a decade. that's something like 7000 of these low-tech, but very efficient life-changers. mexico city has one of the more complex water problems in the world. first of all, it's very far away from any coast or from any body of water. and it's very high up. we're 2200 meters above sea level here. in a valley, any water that you're potentially going to bring into the city, you'd have to pump entirely uphill and like across the sierra, which is virtually impossible. - at least 250 000 people here aren't even connected to the grid. millions more have scarce access to running water, sometimes just for a few hours a day. many, mostly in poor areas, rely on water delivery trucks for their share of drinking wa