I wonder what the cost would be to excavate a 250m diameter, 1km cylinder of rock versus building a large reservoir on top of a 3km mountain. Granted, there ar

I wonder what the cost would be to excavate a 250m diameter, 1km cylinder of rock versus building a large reservoir on top of a 3km mountain. Granted, there are places in the world where it's going to be impossible to find a nearby mountain, but I'd imagine that it's similarly difficult to find places where you can dig out a stable 1km cylinder. Bedrock is not always a single hunk of rock, it can have seams that separate it into separate chunks of rock. And while in some places bedrock reaches the surface, in others it may be hundreds of meters below the surface.