I had my best fort going thus far. It was an almost perfect embark site. It had native gold veins 1 z-level below the surface and many levels down. With that, there were deeper veins of silver among other ores. There was a pocket of magma around 22 z-layers under the surface and at least one adamantine vein starting on that same layer along with plenty of marble to go around. Heck, the only downside was the lack of iron, but I was in no short supply of goblinite (I was pounded on by multiple goblin sieges with the odd human siege or two back to back), so I had oodles of iron armor/weapons that would suit nicely to make steel (picking up all the goblin crap left behind took my dwarves an ice age to collect, holy crap). I even had a spiffy vampire dwarf locked away where my magma workshops slaving away at the forge. He used to serve in my military and the local goblin menace hated him enough to personally declare him an enemy (after looking at legends mode when I looked up this vampire). Guess they didn't like how he single-handedly slaughtered most of their jabberers during one of the sieges. Anywho, I considered holing him up outside in a shack or something for the next siege, but realized he was my only furnace operator (and was multi-talented with blacksmithing and metalcrafting).
So everything was going fantastically. I started digging down this adamantine vein as I was starting to set up a living/working area to accompany my magma forges. There were a couple z-levels this adamantine vein went through that was in my second cavern layer, so I figured I'd build a bridge to secure my fort from the vein. I did this since I had plans to completely mine out the adamantine in these 2 levels so Forgotten Beasties won't get into my fort since there were 2 or 3 of them roaming that particular cavern. 'Lo and behold I found the third cavern layer that was dark and haunted. A bazillion demons, fiends, phantoms, and every other child's nightmare (Yeah, I'd put my money on evil clowns down there) inhabited this place. So, logical decision? Throw the lever, save my fort, and sacrifice my miner. Nope.... I thought I would have time to get that miner out and secure the fort before the demons came through and it was too late by then. The game, ironically, added insult to injury as a necromancer from a nearby tower decided to throw his walking deaders at me. Unfortunately, my last 2 dwarves couldn't live long enough for me to see the demons and their cohorts and the undead fight each other.
So, this raises a couple questions. Are there any hints that might tell me there is "fun" to be had when digging into an adamantine vein (as in, if such vein is peculiarly close to the surface)? And no, I prefer not to use DFHack to peek. Second, I noticed that after I got the bridge closed (too late by then) some of the demons/phantoms/etc somehow got outside the adamantine vein (did not break a hole out if the vein, btw, and were roaming around the magma lake. Even if I closed the bridge, would they have found a way inside anyway?
I know I could have left out the story of my demise, but I had fun seeing it coming to an end. Thanks for answering my questions.