They are the buildings in far western China that haunt the sleep of Uighur Australians. People go in and they don't come out.
"Everyone talks about their absolutely ruined dreams," says Sydney resident Ruqiya of anxiety levels among her fellow expatriates.
In the past 18 months, hundreds of these buildings have spread rapidly across the desert towns of China's remote Xinjiang Uighur Autonomous Region.
The Chinese government has rejected claims that up to one million of Xinjiang's 10 million Uighur Muslims are being held inside. It claims the buildings are education centers that teach the Chinese language to Turkic-speaking Uighurs, part of a campaign to "get rid of the environment and soil that breeds terrorism and religious extremism".
On Tuesday, at a United Nations Human Rights Council review, China will be challenged to provide statistics on how many people are detained without charge, with a call for their immediate release.
A Central Asian people whose presence in the Tarim Basin dates to the 8th century, Uighurs have had an uneasy relationship with Chinese rule for 250 years, scholars say.
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