On 12 July 2015 a satellite swung over the rolling deserts and oasis cities of China's vast far west. One of the images it captured that day just shows a patch of empty, untouched, ashen-grey sand. It seems an unlikely place to start an investigation into one of the most pressing human rights concerns of our age. But less than three years later, on 22 April 2018, a satellite photo of that same piece of desert showed something new. A massive, highly secure compound had materialised. It is enclosed with a 2km-long exterior wall punctuated by 16 guard towers.