Potosí is a city in Bolivia.
South towards Uyuni
Take bus n° 70 or H (Bs. 1,50, they stop in front of the cathedral) to Cantumarca. The bus will go right to the end of the neighbourhood, and essentially of Potosí, where there is a gas station before the road sharply curves and the urban area ends. From there you can hitch a ride to the nearest tranca (toll gate) where cars slow down (10 minutes drive) or directly find a ride to Uyuni.