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.

As cities become more crowded, architects and builders face the constant challenge of creating 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 if something goes wrong.