Wasn't able to think of better name for this, so sorry...
I was toying with world generation parameters. Mostly just removed the erosion, increased a Min Volcanism to 30 and increased the number of volcanoes. I was just trying to find those volcanoes sticking up from more or less flat ground.
So, the generator didn't bring any errors, and was able to create world just after 50 failed worlds. In the world there was one spot that was what I was looking for. Set my embark profile, then begun waiting. I waited around ten minutes, which more or less confirmed that I'm going to deal with one of those unique worlds.
So, when the game launched I immediately paused so that I can study the world on my leisure. Just pausing the game took about 15 seconds, though nothing actually happened inside the game. So, what I got was map clearly made in two separated sections. One section was more or less "normal" in the terms of elevation. Now, the other part, well, there is that huge mountain, the base of it is around the level where the first section has magma sea, around level 100, and then goes up to nearly level 3000. Above it there is empty air levels up to over 4000.
Now that, not all the... "interesting" parts of the map. It seems that in that level when normally the world would end, the map just throws some empty cave down, slowly turning to solid stone every level you go down. There is around six "magma pipes", though not sure if those are just magma sea not changing shape each level. No adamantine, and no caves visible.
I'll post the pictures tomorrow as well as world generating data, embark location as well as the save. Just note, this is a bit modied game, though nothing that would cause this "feature".