So, here's what's going on: I'm turning a small island map that's overrun with goblins, humans, elves and necromancers into an oppressive dwarven empire for adventure mode exploration fun. I've been throwing up some torture towers and dungeons around the map, and am starting to develop a consistent imperial theme to my fortresses. I'm planning on building the imperial palace on a small volcano-riddled grassland island in the calm north, and right now I've got a bridge crew ready to connect the island to the mainland before I begin construction of the palace, on an 8x1 map. Why they decided to ford the wagons across the sea to the island to being work, I'm not sure, but I don't mind so much as I get to begin work in a grassland instead of the rocky wasteland on the mainland.
Now, I could just build a tunnel, it would be dwarfy, but not as impressive enough for the mighty empire. I could suspend disbelief and accept the supreme dwarven skills in architecture that allow them to build large buildings in the air on a single support, but I would like to have proper supports for this bridge sitting in the ocean, as well as a good 3x3 staircase leading beneath the sea for a guard fortress. I've never tried to do something like this for water that is more than one z-level deep, and I'm not sure the best way to go about this. I've considered dumping magma onto the water and sort of carving out a pillar as I go along, and I've considered casting massive obsidian pillars above the ocean and then dropping them in.
Am I missing a more simple method here?