So heres the deal. I know that if your cut off from the surface, I've had traders arrive through the caverns before. And I' fairly certain that migrants have arrived through the caves before too.
Basically, I was looking through an older laptop that I ahve, looking for some old 3D models to add to my previous work portfolio, when I came ran into a series of games I used to play a lot.
Exile. A series of old RPG games made by some indie dev years ago (He also has a remake series Avernum, that I think ima go buy now).
Basic story intro is that the surface empire's mages made a portal and found a way into a massive undergound series of caverns. The empire decides that they're going to turn them into a prison for everyone they don't like, and build a 1way portal into the cave network, expecting that most of the people would just go and die. But instead they live and slowly build a civilization in the caves, and across the few games find their way back to the surface. Loads of fun had by all.
Now I've read that there are ways you can use an adventurer to go into the caverns and change to fortress mode somehow. Found a couple of posts about people doing this back in DF2010, however on following their instruction it generally has caused me severe crashes along with destroying 1 of my longest lived fort files (I've got a backup on another computer, but its like 60 years earlier than I had it up to on here). Tried looking for more recent guides, but generally the search feature just locks onto the word cavern and just blasts me with lots of stories of people getting ownt in the caverns.
Anyone who can point a finger in the right direction of embarking underground would be rather helpful. I've currently just got one going where the second the game started I blasted a hole to the caverns, dug into a wall quickly to make some storage and then boarded up the tunnel to the surface. Ended up as a tight tunneled caverns though so I think i might try another one for a open cavern (safer in some ways, more dangerous in others, loads of !!FUN!! either way)