Electricity powers the life support systems that keep our cities running. Lights, trains, communications, even hospitals rely on this invisible force. Without it, our cities stop working and lives are at risk.

Food: human life can’t exist without it. It’s a city’s most important source of energy. In New York alone, 8 million people consume 10,000 tons of food every day. With demand for food in the city so high, the restaurants and supermarkets must be restocked on a daily basis. Without new supplies, cities will run out of fresh food in a matter of days.

People are the life-blood of cities. Its transport links are its veins and arteries. If they’re cut-off the city will die. Thousands of people work every day at making it possible for city dwellers to be where they need to be, when they need to be there. It’s an endless task demanding brain and brawn. Without the army of drivers, diggers, planners, thinkers, our great cities would come to a halt.

For centuries cities have been built near a fresh water supply. Without it we’d be lucky to live three days. But this most basic human need can be deadly too—get the supply wrong and it can poison us, get too close and it can drown us. And with every passing year the challenge of providing water to billions of people becomes harder and harder.