Cobalt is a town founded on silver mining. In 1903, while constructing the railway, silver was discovered, and a town was born. 400 mines later, there isn't much happening in Cobalt today, but there's LOTS to explore.
Additional information available on Wikipedia.
It doesn't seem to matter how many times you go to Cobalt, you'll keep finding something new to explore, and you'll never have enough time to do it.
I'm sure I'll be returning many more times before I'm done.