I recently decided to try out the '12 civs who all want to get you' mod, and I decided to settle next to a lake with a volcano island within embark range of the shore. When I embarked, however, I discovered that the volcano is BELOW WATER LEVEL! The water instantly leaked into the volcano, causing 50 or so cave ins as it created a (sadly underwater) obsidian cap over the volcano. This was just the start of the crazyness. The cave-ins allowed me to see the magma sea, all three cavern layers, and HELL. Because THE MAGMA SEA WAS CONNECTED TO HELL! And Hell was connected to all three cavern layers! Hell itself was split in two, with half of it being on the bottom level and half of it being 18 floors below the higher half. Not only that, but the top layer of the upper hell was only FIVE Z LEVELS below the surface. FIVE. And the upper hell also had a magma tube in the middle of it. Magma immediately started flowing into hell. Unfortunately, it is all flowing into the glowing pits, which just absorb the magma. This is causing the volcano to gradually drain, and slow down the FPS a lot while it does so. I suspect Fun will commense once I try to get some magma, as it is now connected directly to demonland.
*update*
Just before posting this, I smoothes a portion of the (not slade or addy) wall of upper hell. The horde of demons appeared, but so far have no path into my fort.
I took screenshots, but I don't have anywhere to post them :/