I just found a "Moriaesque" cave/mine.
What seemed like the usual cavern corridors with different branches and small halls here and there actually lead to a very BIG hollow hall with galleries on each sides, going from the "roof" to the bottom. And the bottom is very deep. As in, you can see lava from the top of the halls. That place's a deathtrap. Critters spawn from every direction because of the many galleries it has.
I'm halfway done placing torches in it to avoid more baddies disturbing me. When I first discovered the place, I immediately launched http://www.youtube.com/watch?v=NUIZvAe3RBg in iTunes as I battled the creatures, avoiding pitfalls, placing torches and looking my way down on the lava falls below.
It will be an awesome place to "colonize". As I've decided this time to live exclusively underground. I may be living in a claimed or unclaimed territory, I do not know. All I know is that I'm somewhere close to spawn. Deep, deep underground.