While land and sea creatures rely on air and the oxygen it contains to breath and survive, winged beings can actually live in the invisible element. Some, such as the sooty tern and Alpine swift, can spend months at a time in the air - even sleeping in it. And they are joined by many other skilled masters of the art of flight.

But life up high is not without danger. One threat comes in the form of gases pumped into the environment from chimneys and exhaust pipes. To a certain extent, humans can protect themselves from bad air, but animals can't close windows or wear gas masks when the going gets tough. Their only defense is flight. Air pollution poses a major challenge in the world's sprawling megacities, and must be tackled if people, animals and plants are to thrive.

This interactive graphic will lift you up into the realm of flight, where you can explore some of the creatures that populate this invisible element.