Controversial online currency Bitcoin has made headlines over the past couple years, mostly due to the fact that it’s not regulated by a government authority. Some thought it was a fad, but it doesn’t seem to be going away.

Last Fall, Motherboard visited a massive and highly secretive Bitcoin mine located in a repurposed factory in rural northeast China. For the feature, they gained access to the inner workings of one of its six operating sites, where some 3,000 miners generate 4,050 bitcoins a month, the equivalent to $1.5 million in real world value.