Accompanied by tinny taped music and overall-clad workers, Rio Tinto Group executives and Mongolian officials gathered a kilometer beneath the freezing Gobi Desert earlier this year to open one of the world’s richest underground copper mines.

It was a celebration four decades in the making.

Oyu Tolgoi, in southern Mongolia just north of the Chinese border, is key to Rio’s efforts to move beyond its dependence on iron ore and expand in copper, the metal that underpins the clean energy transition. It’s also a vast deposit whose corporate, political and technical vicissitudes offer a glimpse of the red metal’s troubled future.