RE:Retiredgeo..........................Barrick Gold's Hemlo mine started as an open pit before going underground. Dome mine also started as an open pit. I worked at an open pit asbestos mine in Quebec that went 600 or so feet deep and then went underground. It is all about economics. The deeper you go the stripping ratio rapidly increases. Try an visualize being in a pit 10feet deep and you want to go 10 feet deeper. You have to remove all the side rock all around the existing pit to permit you doing this safely. Notttoo much, however if you are 300 feet deep, now look at how much rock needs be removed in order to excavate, and the gold in those last 10 feet needs to pay for all the extra rock removed. You can see that the stripping ratio at that depth is huge, thus although mining UG is more expensive per tonne the stripping ratio makes it more economical.