Hard rock mining is pretty insane when you look at the history. In california the Empire mine hit ~3.3km deep in the 1950s before closing due to costs. That's a

Hard rock mining is pretty insane when you look at the history. In california the Empire mine hit ~3.3km deep in the 1950s before closing due to costs. That's about 2.4km below sea level. https://en.wikipedia.org/wiki/Empire_Mine_State_Historic_Park https://en.wikipedia.org/wiki/Empire_Mine_State_Historic_Par...