I will admit I am not the smartest person in this big gene pool.
So I may have misunderstood this picture Highlands has on their site.
This is a picture of the Bingham Canyon Copper Mine, the deepest mine of this type in the world.
And as you can see, they way Highlands has shown it, THEIR mine (and it is a mine, NOT a quarry, check out the definition) will be deeper? (See the orange line)
Did I get it wrong?