This game is too much for my heart. First, I found out that a relatively small tunnel cave went all the way through the mountain, coming out in one or two places under the giant Moria-like cave entrance at the base of my main fortress's cliff. Then, I realized it was getting dark and had to hightail it all the way around the mountain back to my base shelter (which may also be pretty close to spawn...it's down there for safety and also mining).
THEN, I went down the Endless Stair I had constructed, to the bottom of my mines, where I had found the dungeon, and started exploring. I found what may have been my first random cave zombie, as I was exploring some tunnel I hadn't walled off far beneath my mountain base cave house, and I tried to turn and run, getting stuck on rock or something, before remembering I had a sword and killing it. I also heard some scary groan/growl elsewhere.
And THEN I found that the tunnel there apparently connected to a deep sand/rock sinkhole near a waterfall/ocean cave I had started to light up previously. It's going to take me ages to light all this stuff up, but at least I've found a little bit of iron in these tunnels. It doesn't seem to occur more than a few blocks at a time so far for me.
I want to start farming grain and cacti soon. It would be nice if you could farm flowers...I haven't found any papyrus to farm yet.