People should dig mines in worldgen. For dwarves, it can be like a basement addition to their mountain halls; for goblins and humans it would probably be out somewhere farther away from their settlements. Mines could pierce caverns, allowing the civs access to creatures and plants from that layer if they didn't already have them. Mines could pierce the second and third cavern layers, eventually finding the magma sea, and gaining access to species of those cavern layers and (maybe) any tameable magma creatures as well. Striking magma would promote the creation of a large metals industry there, as the heat of magma would eliminate the need to burn wood for fuel and lower costs. Once a mine such as this got big enough, it could start to house people, maybe on the surface or maybe in the caves or in rooms carved into the cavern walls or whatever; it would probably vary by race.
However, it wouldn't be all peaches and cream; monsters from the depths could come up and attack the mining settlement, killing people and cutting off supply to ores and cave plants. This would be an interesting quest: "Kill the trolls occupying the mine, so that we can have access to our goldsmithing forges and dwarven whiskey again!"
If a mine went deep enough in a area with adamantine, the civ might have access to some adamantine, although not enough to sell to the dwarves as toy boats or to outfit every rank-and-file soldier with it--probably just royal guards, statues for the king, MAYBE selling to the dwarves for insane prices if they've shown themselves to be trustworthy. A civ's discovery of adamantine would be kept secret as long as possible, but word WOULD get out eventually, leading to wars fought over a few bars of bluish metal. Maybe civs could find and raid demonic fortresses as well, or at least hire adventurers to investigate the strange place.
The big thing would, however, be releasing demons. If a civ mined enough adamantine or had the sword removed from a demonic fortress, the mine would be taken by demons, who might then launch attacks on nearby civs. Adventurers could try to reclaim mines, at least once we can have them send along masons to seal up the cracks in the demons' home, and a demonic invasion would be an interesting possibility for fortress mode...Hopefully, not too many would come at once.
Overall, discovering adamantine shouldn't happen very often, but maybe requiring mines to be made in the same region tile as an adamantine vein or demonic fortress would reduce the chances enough. It depends on how often and quickly new mines are dug.