Hundreds of Tibetans protesting against a Chinese dam were rounded up in a harsh crackdown earlier this year, with some beaten and seriously injured, the BBC has learnt from sources and verified footage.
Such protests are extremely rare in Tibet, which China has tightly controlled since it annexed the region in the 1950s. That they still happened highlights China's controversial push to build dams in what has long been a sensitive area.
Claims of the arrests and beatings began trickling out shortly after the events in February. In the following days authorities further tightened restrictions, making it difficult for anyone to verify the story, especially journalists who cannot freely travel to Tibet.
But the BBC has spent months tracking down Tibetan sources whose family and friends were detained and beaten. BBC Verify has also examined satellite imagery and verified leaked videos which show mass protests and monks begging the authorities for mercy.
The sources live outside of China and are not associated with activist groups. But they did not wish to be named for safety reasons.
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The protests, followed by the crackdown, took place in a territory home to Tibetans in Sichuan province. For years, Chinese authorities have been planning to build the massive Gangtuo dam and hydropower plant, also known as Kamtok in Tibetan, in the valley straddling the Dege (Derge) and Jiangda (Jomda) counties.
Once built, the dam's reservoir would submerge an area that is culturally and religiously significant to Tibetans, and home to several villages and ancient monasteries containing sacred relics.
One of them, the 700-year-old Wangdui (Wontoe) Monastery, has particular historical value as its walls feature rare Buddhist murals.
The Gangtuo dam would also displace thousands of Tibetans. The BBC has seen what appears to be a public tender document for the relocation of 4,287 residents to make way for the dam.
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China is no stranger to controversy when it comes to dams.
When the government constructed the world's biggest dam in the 90s - the Three Gorges on the Yangtze River - it saw protests and criticism over its handling of relocation and compensation for thousands of villagers.
In more recent years, as China has accelerated its pivot from coal to clean energy sources, such moves have become especially sensitive in Tibetan territories.
Beijing has been eyeing the steep valleys and mighty rivers here, in the rural west, to build mega-dams and hydropower stations that can sustain China's electricity-hungry eastern metropolises. President Xi Jinping has personally pushed for this, a policy called "xidiandongsong", or "sending western electricity eastwards".
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The Chinese government has long been accused of violating Tibetans' rights. Activists say the dams are the latest example of Beijing's exploitation of Tibetans and their land.
"What we are seeing is the accelerated destruction of Tibetan religious, cultural and linguistic heritage," said Tenzin Choekyi, a researcher with rights group Tibet Watch. "This is the 'high-quality development' and 'ecological civilisation' that the Chinese government is implementing in Tibet."
One key issue is China's relocation policy that evicts Tibetans from their homes to make way for development - it is what drove the protests by villagers and monks living near the Gangtuo dam. More than 930,000 rural Tibetans are estimated to have been relocated since 2000, according to Human Rights Watch (HRW).
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Multiple Tibetan rights groups [...] argue that any large-scale development in Tibetan territory, including dams such as Gangtuo, should be halted.
They have staged protests overseas and called for an international moratorium, arguing that companies participating in such projects would be "allowing the Chinese government to profit from the occupation and oppression of Tibetans".
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