Well, it seems the Greater mines is only a front door, because I decided to dig deeper and I suddenly realize that I can see the sky. Looking through the new window I made, I come to realize that Greater Mines has a cliff face on the other side.
It's not a total bust, I spotted a cave on the other side. I think I have reason to make use of the export of wood from Starter Mines (thanks to it's tree farm). The living quarters will be a series of suspended huts (possibly treehouses (material storage and housing)) linked by bridge. a bridge of sorts will be made to connect the entrance and housing to the nearby cave to serve as a forge/office or as another mining location. Heck, it could even work as a starting point/station for a minecart subway.
Unlike Starter Mines, this place will go deeper, at least by 2 more levels, and should have additional mines. Seeing as there's exposed material on the backside of GM's rock-slant, I could build some scaffolding which could also work as a hunter/sniper post. Too bad I didn't survey the area before I carved the first parts of the GMs. Oh well, I can make do with what I got. I can keep myself supplied with everything thanks to my system I have setup; and surprisingly, the GM location seems relatively safe, for now.