Yeah, pretty sure it's impossible. It's a bug. I looked through the script files - interestingly, nowhere in the script files is the command given to clear the North Mines - it's given for the other three, but not North Mines.
If you look at how the zones are numbered, the hint book has it like this:
38: Eastern; 39: Core; 40: Northern; 41: Western
In terms of the script file numbering (and the numbering of most of the code):
37: Eastern; 38: Core; 39: Northern; 40: Western
To clear a zone, the script uses a function clear_zone, so to clear Eastern Mines for example:
clear_zone(37)
Each of the four files associated with mine zones that use this command, call it for 37, 38, and 40. This is unusual, most zones call "clear_zone(ME)" to clear themselves. The mine zones clear each other all at once. But 39 (Northern Mines) is never listed anywhere, and there's no clear_zone(ME) in any of the Northern Mines script files either.
http://www.spiderwebsoftware.com/geneforge/techsupp.html
It says there for 1.02 bugfixes: "Can now clear the northern mines and inner crypt."
So what happened?