I still like my magma easily accessible so my latest fort is right smack on top of a volcano in a tropical broadleaf (jungle) biome.
To my delight the pipe is not only open to the air but juts about 5 z-levels above the 'north' face of the hillside. The 'south' face of the hillside rises to 2z above the top level of the pipe, and since my 7 started ON the north (cliff) edge of the pipe, sans wagon of course, I just dug right into the south side to start.
Another salient point about the embark was that the pipe was only visible down about 60-70 z levels of a total of 130 or so. After that it went 'black' and I had no idea if that's where it ended or what.
Anyway, about a year after embark it was time to clean up my entrance and one of the first tasks was removal of a small 1z hill on the south slope, right above the top of the pipe. Out of laziness I just designated the entire thing as ramps and let my two miners go to work. As you have probably already predicted, "A portion of the cavern has collapsed!" and one poor miner was knocked back into the pipe.
I unpaused the game, and went on managing my fort. A minute or two later I get the next announcement "You have discovered a vast magma sea.", followed in short order by "You have discovered [REDACTED]! Praise the miners!"
So it appears that Urist's last communication with the hive mind was detailed geological information on the volcano. The strangest part was after the [REDACTED] announcement I realized I couldn't find the miner anywhere down in the depths. Also, he wasn't dead! I zoomed to him from the Unit list and he was still at least 20z above the top of the magma sea. I guess as soon as he passed the last z-level that was revealed on embark the game chose to show me the rest. I unpaused and a few merciful seconds later he hit bottom and promptly bled to death.
Normally, this early in the game I would just save scum since it was really just a boneheaded move on my part that caused his death. This time I decided to play on, recognizing the brave sacrifice made by Urist and his last actions of geological survey.
Unfortunately I then took WAYYY too long to get my farming up, forgot completely about harvesting plants, and everyone died of thirst about 3 seasons later. I guess that fort was just doomed.