There are around one million people airborne at any one time. But what goes up must come down - and bringing all those people safely back to earth depends on complex global networks and astonishing technology that stretches our ingenuity to the absolute limit.In this programme, science broadcaster Dallas Campbell and Dr Hannah Fry explore just what it takes to bring the citizens of the sky back to the ground. Dallas has a front row seat when he is in the cockpit with one of just 26 pilots in the world qualified to land at Paro, Bhutan. Meanwhile, Hannah meets up with the air traffic controllers who, at some times of the year, deal with over 1,000 flights a day arriving at Atlanta - the busiest airport in the world. She also visits Bangor Airport in Maine where they are always on standby - there have been over 2,000 unscheduled landings in the last decade alone.With the city in the sky predicted to double in size in the next 20 years, in this last episode in the series, the team also find out what the challenges are and what the future of aviation might look like.