They are elevations above a zero mark set at 10,000 feet below sea level. That was originally done to get around the confusion which could arise with a mine that operated partly or wholly below sea level.
That plan appears to have something to do with water pumping arrangements.
thanks for that Caiman, I don't think I'd have worked that out for myself. The section of Cults mine shown is the deepest innermost section that is completely underwater these days, as is much of the eastern part of the mine.