What at first glance looks like the ruins from some ancient indigenous settlement in fact dates only from the turn of the 20th century. Created by Bolivian Hunachaca Company (one of the richest silver-mining operations of the late 1800s), the site was used as a foundry and refinery for raw material shipped in from the Pulacayo mine in Bolivia, and once employed more than 1000 workers. You can take photos of the ruins, but are not allowed to enter.
The small museum in front of the site has exhibits on natural history, mining and indigenous culture, as well as a four-wheeled vehicle dubbed 'Nomad' (an early prototype for the Mars Rover) thrown in for good measure. To get there take bus 102 or 103 from the center to Calle Sangra (CH$500).