If you've not yet played minecraft or not yet gotten into deep spelunking, let me warn you: you will get lost. Not "oh no I ran too far and night is coming" lost. I mean something more like "I've gone through this same cave ten times already and I keep picking a different path but I keep ending up back here" lost.
I'd been doing some cave running, searching for more diamond (I was too shallow anyway) and more iron, since despite creating 16 ties per craft, rail lines are still pretty expensive. Sometimes I'd hear something in the wall, so I'd dig over and find a new cave system. So I'd explore that one too. And then I'd see some dirt or gravel and since it's so quick to remove, it seems sensible to clear it out. I've found iron and coal this way. Or sometimes new caves. Notice a trend?
By the time I was entirely lost, I think I'd somehow linked all but one tunnel. Before this I had a good system. Torches marked paths I'd cleared and explored, safe areas. Cobblestone barriers indicated that I'd fully cleared a branch and had blocked it off so that I could reclaim the torches and avoid spawns ambushing me. This meant that I would always run to a new, unexplored area the next time I went searching. But once everything was linked, suddenly I had a half-dozen entrances, some of which turned out to lead almost directly to each other, resulting in the endless cycle of running down one tunnel, searching for a new area, and running right out another.
Then I figured out how to simplify things a bit more: Place TNT in areas where I think two tunnels are close together and just blast until there's one giant room. It's the American way: reduce complexity by blowing up everything that confuses you.
But I still need to figure out why three different paths lead to the same underground river from slightly different directions. I'll need more gunpowder.
And my rail system needs some switches so I can pick between my home and my furthest fort. That fort needs repairs after a creeper blew out the wall one morning and then a skeleton shot me up. Speaking of rail, I'm working on moving it to an underwater/underground tunnel, so hopefully creepers won't keep blowing up sections of the tracks when fighting at my bunkers. Creepers are assholes.