In 2017 there are hardly any corners of the globe that planes don’t reach.
But one flight path pilots won’t make is over Tibet.
The autonomous region sits in western China, sharing the border of Mount Everest with Nepal.
Tibet has been nicknamed the ‘Roof of the World’ thanks to its towering Himalayan peaks.
It’s these peaks that prevent planes from flying over the region, according to a thread on Quora.
One user posed the question: “Why don’t planes fly over Tibet?”
Airline worker Tim Hibbetts explained the threat posed to aircraft from these extremely high mountain ranges.
He said: “There’s an issue with the very high mountains and losing an engine, but under most conditions, airliners can still fly higher than 20,000ft.
“That’s not going to help over Mount Everest, but with the gradual nature of losing the altitude, it’s rarely a big deal.”
Tim said the problem is related to oxygen masks, which only have about 15-20 minutes worth of oxygen for each passenger.
If there was a decompression in the cabin, the plane would have to descend lower than the height of the mountains to keep passengers breathing.
He explained: “There are flight rules in place that force operators to be able to descend to 10,000ft prior to running out of emergency oxygen.
“Obviously, this part of Asia is a huge region and there would be a large swathe that would be impossible to escape so quickly.
“On top of that, there would be several points that would require the airplane to climb even higher to get to a suitable alternate field, exacerbating the situation.”
Mathematician Yasha Berchenko-Kogan agreed flying over the Himalayas is a “bad place to be in an emergency”.
He added: “If there’s a loss of cabin pressure, the airplane is supposed to descend to an altitude where there’s enough air to breathe. You can’t do that over the Himalayas.
“If you lose an engine, you are also forced to descend, but over the Himalayas that means crashing into a mountain.”
Due to the remote geography of Tibet, planes also don’t have much need to fly this route.