Anyone know of a reliable way of navigating these things? I've thought about using trails of redstone dust, but it seems a bit labour intensive and won't help much with irregular vertical spaces. I got turned around twice and it took me near to an hour to find my way back out again.
When I don't know how to get back through the tunnels, I usually just dig upwards until I get to the surface
I sometimes use signs to indicate which direction I went when there was a split in the tunnel. I've also started to wall off sections that I don't yet want to explore. I leave a thin opening so I can see it's not a mob spawner later on.
^In that instance, digging up wouldn't have been feasible. For one thing I'd been doing a lot of mining and was down to just half a stone pick and it wouldn't have lasted all the way up. The cave system was down deep, I had broken into it while digging my subway tunnels, which were only 20 or so blocks above the bedrock, plus I'm pretty sure I was under a lake.
I do make a habit of walling off tunnels, but usually only after I've mined it out. Perhaps I should to both and mark it somehow to distinguish between a dead end and an unexplored passage. Either way, vertical areas are still always a tricky prospect. I think I get into the most trouble while rushing around, dropping torches in all the dark areas so I don't get mobs spawning behind me. I once got ambushed but FIVE creepers, two zombies, a skeleton AND a spider. No exaggeration, there were loads of the buggers and it cost me the five diamonds and 20 iron I was carrying since I couldn't find my way back from the spawn in time. Been paranoid about lighting my way ever since.