Chinese frontier soldiers patrol on Pamir Plateau in a helicopter in Northwest China's Xinjiang Uyghur Autonomous Region. (Photos: 81.cn)

The patrol route is 5,000 meters above sea level on average. The zone is called the "dead zone" for flights due to its treacherous conditions, freezing temperatures and low oxygen levels.

The unique scenery is caught out of the helicopter's window.

Since the first air route was opened up in 1990s on the plateau, frontier soldiers have made 4,000 hours of safe flights and have opened up dozens of air routes.

Soldiers carry boxes of supplies that are delivered to their sentry post with a helicopter.

The Khunjerab Pass on the China-Pakistan border is also under the guard of the frontier soldiers. The pass, the highest border-crossing in the world, is also one of the busiest passes in China. (Compiled by Huang Jingjing)