In the "dwarves are extinct" world, I breached the cavern layer to see what was down there, and was terribly disappointed. Bloodthorn trees, sporadically scattered over hard stone floors. No water. Not bushes. No floor fungus. It became immediately clear why dwarves went extinct.
I saved the game, and promptly regenerated the world with 3 cavern layers, on the same world and history seeds, but with civs pushed really really high. Dwarves survived worldgen for 1050 years. Elves and dwarves are at war. Goblins and dwarves are at war. Humans have managed to become necromancers and build towers.
I embarked on a nice little volcano. Sadly, it was obscenely high. (52 z levels!)
Rather than waste several days of my life, I decided I wanted some fun this tim around. I whipped out dfhack's liquids, and cast a humongous 52 zlevel obsidian spire to eliminate the volcano's undesirable features.
Dwarves have cleared the first level, and set up shop. 2 years now since embark, things are going well.