A new railway has opened linking Tibet to China. China says it is an engineering feat, but critics say it will speed up the undermining of traditional Tibetan culture.
The 1,100km (750-mile) Qinghai-Tibet railway has been built through some of the most difficult terrain on earth, and rises at one point to 5,072m (16,600ft) above sea level.
Because of the region's extreme temperatures and unstable permafrost, long sections of track have been elevated to improve stability.
China says the $4.2bn (£2.3bn) railway will bring trade, jobs and tourists to one of its poorest regions, and help address economic imbalances between the country's east and west.
But Tibetan groups and foreign critics say the railway's real aim is political, as a symbol of China's administrative and military control over a contested border region.
There are fears the railway will speed up the immigration of ethnic Chinese into Tibet, threatening its distinct cultural and religious identity.
Critics also complain that most of the jobs created will go to ethnic Chinese, as Tibet's under-funded schools leave local people poorly educated and unable to speak Chinese.
China says environmental concerns were given top priority. It cites the Tibetan antelope, and says construction was suspended so as not to interfere with the animals' migration.
But longer-term environmental costs - especially those resulting from population and economic growth - will be harder to dodge.