Raw adamantine at layer 25 is no big deal; actually, that's pretty low as compared to my experiences. As for the magma sea, when you say "8 blocks below sea level" (the common term around here is "z-levels" by the way. I don't mean anything negative by that; just sayin') do you mean the red number at the top right says negative 8? or that the black number at the bottom right says negative 8? The top right value tells you how many z-levels of non-sky are above the cursor, wheras the bottom right tells you the absolute altitude of the currently visible z-level (though the world technically extends to -25. I think it's -25 at least.)
I'm guessing the "8 blocks below sea level" comes from the top-right number, which just means the ground in that area was really far down, which isn't too big a deal other than you don't have much vertical space to work with. As for the adamantine, if you're not seeing the top of it, it may go up farther than you see.
Ah I see, I guess I settled in a very deep area of the map. It showed as a -8 on the upper red colored depth meter (I actually didn't notice the bottom black one until you mentioned it), while on every other map I've played I usually had to go down as far as -100 -140 just to reach the magma sea.
I was going to make a screenshot of the pit I found, and the addy veins, but apparently as I was prepping for the screenshot 2 trolls went into my VERY immature fortress, which only had 1 migrant wave so far and only a single row of cages as traps, and crushed my dwarves into a thousand tiny pieces.