Yerington (Weed Heights), Nevada, USA. This is the fate of any abandoned mine that's deep enough to get below the water table--it floods to the level of the water table! Pit lakes are a serious environmental problem in lots of areas, including some other places in Nevada. What often happens is that, in contact with water and oxygen, sulfide minerals oxidize to sulfuric acid--and then, as the water gets more acidic, it leaches lots of metals into solution, where they make the environmental problems even worse. This is the mechanism behind so-called "acid mine drainage." Fortunately, the Yerington pit lake isn't particularly acidic, because less sulfide ore is exposed than in lots of other pits. It does have selenium levels high enough to exceed drinking water standards, though. You wouldn't want to water-ski in it!

Acid mine drainage is occurring, however, at the Leviathan mine in California, about 50 miles or so west:
www.ipernity.com/doc/289859/18665269/in/group/308219/self
That mine is in fact a Superfund site.