Potosí - The World's Highest City
Not only is Potosí higher than La Paz, during colonial times Potosi was the most populated city on Earth! There was so much silver in the mines at Cerro Rico that the Spanish (and Bolivians after them) have mined it for over 500 years. Tin overtook silver to become Bolivia's prime export during World Wars I and II and Bolivia's wealth increased dramatically at the time. As of only about 15 years ago, oil and natural gas from Eastern Bolivia (Santa Cruz and Tarija) have replaced tin to fill Bolivia's coffers.
In Potosí the two most interesting things are the mines and the Casa de la Moneda. When I was little women were not allowed into the mines. Only men could go in. The miners believed that El Tio (the devil) would cave the mines in if women entered. Working conditions have not improved at all over the past 30 years and although tourists can now enter some of the shafts, I don't really suggest going too far in. Safety from falling rocks and cave-ins is a problem even for miners today, as they dynamite without discretion. But the other problem is the silica. Miners have an average life-span of 45 years because they spend their entire lives breathing in the invisible silica particles that are like fiber glass that cuts away at their lungs.
The Casa de la Moneda (the Spanish MINT) is where the Spanish turned much of the silver into imprinted silver coins before sending shiploads of them to the King of Spain. It's an extremely interesting museum now.