Heh, I'm afraid I don't exactly know how to do that. Is there perhaps a good tutorial on worldgen to be found? Or, more specifically, how valid are the 40d worldgen tutorials on the Wiki for 2010?
Oh, sorry.
Are you familiar with how to export World Gen information and use it in Advanced World Gen?
If you are, skip the spoiler. If not:
If the world has an existing fort, start the save and go to Export Local Image. Deselect all but one level of the fort with Enter. You don't have to, but it speeds things up considerably and you still get all of the information you need. Hit 'e' and wait a bit. Go to the DF folder and find the text file that says World-Gen-Param. Open it and copy everything in it. You might want to change the title that says something like Large Island or Small Region or whatever to the name of the world in question or something like Scorching Mountains.
If the world doesn't have an existing fort, you can just go to Legends mode and do this by hitting 'p'. Everything from there is the same.
After you copy the information, go to the Data\Init folder and open the World Gen text file. Paste the information into it at the bottom.
Now. Go into Advanced World Generation and move down to the bottom entry. This should be the information you just posted. Hit 'e'. Now, scroll through the data until you reach the cavern settings.
The ones we're interested in here are:
[CAVERN_LAYER_OPENNESS_MIN:0]
[CAVERN_LAYER_OPENNESS_MAX:100]
and
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100].
The first controls the size of individual caverns while the latter controls how many different sections the cavern has.
To guarantee large caverns, set [CAVERN_LAYER_OPENNESS_MIN] to a fairly high number, such as anything within the 90-100 range. Leave MAX at 100.
Then you have two options on the latter and still get fairly large caverns. If you want an absolutely massive cavern system, then set [CAVERN_LAYER_PASSAGE_DENSITY_MIN] to 90-100 as well. Again, leave MAX at 100. This creates so many passage ways that they all combine together in one big cave. The upside is that you can often wind up with one cave that plunges across all three cavern levels at once. The downside is that you can often wind up with one cave that plunges across all three cavern levels at once. This is really hard to defend as there are far less natural chokepoints to build walls and set up traps and there are third cavern level creatures that will end a young fort very, very quickly.
The other option is to set [CAVERN_LAYER_PASSAGE_DENSITY_MAX] to a really low number. Say, 0-10. This time leave MIN at 0. This will create a lot of large passages with narrow connecting tunnels, which is probably better if you're still learning the military and the game in general. I think the caverns look better this way myself as well, but your mileage may vary.