There are around a million people airborne at any one time and keeping that number of people safely aloft depends on complex global networks and astonishing technology. In this programme, science broadcaster Dallas Campbell and Dr Hannah Fry explore just what it takes to keep this city in the sky safe between take-off and landing. Dallas discovers how pilots find their way across thousands of miles of sky in the dead of night. Hannah meets up with the air traffic controllers who are responsible for the busiest airspace in the world - over Atlanta in south east America - and reveal just what is involved in co-ordinating the 100,000 flights that cross the globe every day, while avoiding collisions.