My Cathedral of Blood, dedicated to Armok, built at around z-level 30 deep underground, on a survival multiplayer server. All materials were gathered normally (The glowstone was the hardest), as well as the entire space being carved from solid rock/gravel/dirt, which took HOURS upon HOURS, since I did it alone. Tens of thousands of blocks.. I forgot the exact dimensions, but its massive.
Sadly, its not finished, although the part shown IS. Behind/below where I'm standing in the screenshot (on a ledge), was to be the main alter, and directly behind where I'm standing, behind what was to be a large glass window, was the priest/high priest's quarters. Neither the alter or the quarters were finished, although they were 70% dug out.
Sadly, the server it was on closed down, so all I have left of it is this screenshot and one of it being half-decorated.
The altar was going to have large amounts of gold and iron blocks, glowstone, and of course, magma-fountains. The Door you see on the far side of the original picture was controlled by a server-addon that used iron blocks to control the disappearing of blocks. They removed the feature for non-admins when pistons came out, which made my door completely useless (pissed me off to no end).
I had also planned on making a redstone contraption to allow the sacrificing of people stupid enough to pull the lever near the alter, but that wasn't finished.
Whatcha guys think? This was entirely done using survival mode, and just shows how little of a life I have past DF, Minecraft, and college.