The air pressure is also now closer to what is normal in Denver, Colorado -- the mile-high city at 5, 280ft -- than the traditionally higher, and more mountaintop-like standard of 8, 000ft above sea level, sparing fliers from the mild altitude sickness that can be experienced when flying a long distance.

BBC: Increased comfort in the sky

It traverses oxygen-starved, earthquake-prone terrain that for most of the way is over 4, 000 metres (13, 120 feet) above sea level and rises to 5, 072 metres not far off the altitude of Mount Everest's base camp.

ECONOMIST: Tibet is linked to the rest of China