Electricity powers the life support systems that keep cities running, from lights to communications. Without it cities would stop working and lives would be at risk.

Food is a city's most important source of energy, and without new supplies they will run out of fresh food in a matter of days. Learn how cities master replenishment.

As cities become more crowded, architects and builders face a constant challenge: how to create new spaces in cities where every plot has already been developed.

Transport links are a city's veins and arteries and if they're cut-off the city will die. It's an endless task demanding brain and brawn.

For centuries cities were built near a fresh water supply. 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.