Potosí was, for a century, the richest city in the world, and the Cerro Rico above it is the mountain that paid for the Spanish empire. It is still worked, by cooperatives, in conditions that have not modernised much.
The mine tours are the reason most people come, and they are not a museum visit: you go in with working miners, buy them coca and dynamite at the miners' market on the way, and the tunnels are hot and low.