In my current fortress, I have a 20x20 obsidian farm. 8 bridges controlled by 1 lever similar to the vertical cross section below. When the bridges are open, they block off the supply. It was a little tricky to create the farm without poking any extra holes.
Magma Supply | ____7/7 Magma____ | + |
+ | Open Air | + |
Water Supply | ____7/7 Water____ | + |
+ | Open Air | + |
+ | Casting Area | [door] |
I had forgotten that 7/7 liquids don't all fall at once. So, some of the magma and water ends up mixing mid-air, and then caves in. I ended up with pockets of 7x7 water mixed in the obsidian, and some obsidian on the level above the casting area that seems to be unsupported yet didn't cave in.
So, there was 7/7 water on 2 of the tiles behind the door. I figured the only way to clear that water would be to get a miner on top of the obsidian to dig some tiles out and get the water to evaporate. I know I'm not going to get a second chance, because once the 1 obsidian tile is mined out and fills with water, the dwarfs won't go in.
Anyway, I herd all of my miners into the entryway so that they can all rush in at once to increase my odds of getting someone on top, so that I don't need to poke any extra holes in my nice clean farm. I designate the tile next to the door to be dug, and a bunch of tiles on the floor above to be dug. They all rush in, all of my miners makes it on top. It seems like my plan is coming together. The moment they start digging, everything caves in. Not a single miner made it out alive.