At any given moment, thousands of planes are in the sky.
More people are flying than ever before, and on popular routes, such as Melbourne to Denpasar, or London to New York, multiple planes will fly the same route at once.
Pilots will make the most of jetstreams where they can, which are high-wind passages than can shave hours off flight times.
However, since most of them are only 5km high and 160km wide, it means aircraft become concentrated into what's really a tiny portion of the sky.