Is this possible?
a "lake embark", ideally 3x3 but possibly 4x4 with a central lake at least 5Z deep, flat or mildly hilly dense conifer forest surroundings with at least 1-2 layers of soil. Bonus: coal and metals. Extra bonus: volcano at bottom of lake (with obsidian "cap").
...in a world that is as small as possible and has 1-2 civs of each (dwarv, elf, goblin, human), not overwhelmed by towers/necromancers. World must have steel (somewhere, not necessarily on the lake embark, just able to import it from other civs). Bonus: world conditions are met with history over 250 years (e.g. civs don't die before). Extra bonus: world has at least one ocean edge.
Probably possible, from what I've seen.
Essentially just create a world that favors conifer forests and lakes and you should find it, eventually.
I've seen underwater/below-embark volcano embarks in the ocean and lakes and they are.. problematic.
The water doesn't flow until the moment of embarkation, so.. it won't actually seal itself without going through hundreds (thousands?) of collapsing messages. And even then, it takes a really long time, because all of the obsidian that isn't adjacent to a wall keeps falling in to the SMR sea. And then reforming, and then falling, and reforming, and falling, etc.
I've even seen cases where they never seem to seal, because of this issue. I mean, it's interesting, but I don't think you'd want to seek it out, intentionally.
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To get the rivers and lakes to form and flow just make sure there is at least 8 Z-levels of variation in elevation (100 to 108, for example).
If you want all civs, you won't be able to have exclusively mountains + lakes, because elves and humans need forests and plains, respectively.