So after reading about the Oceanside succession game on the wiki, (quote, "The basic goal is to create an underwater city of clear glass (think Bioshock)."), I thought, why not build a dome above the water first and then sink it in the ocean? Kind of like the ship/city on Stargate Atlantis.
It would have to be made of natural stone walls because I could be wrong but I'm pretty sure constructions deconstruct upon impact. Unfortunately that rules out building it from glass (a pity); it has to be done with cast obsidian.
I decided to try a "small" scale prototype first, just big enough to lock a few dwarves inside, Pull The Lever, and see if anyone recovers consciousness after the vessel strikes the ocean floor!
And so I present to you, the dwarven Obsidian Bathysphere! Screw making pirate ships, I'm making a submarine. Here's the plan (side view):
XXXXXXXXX
X @ @ D
XXXXXXXXX____SSS
Legend: X = solid obsidian wall, __ = constructed floor, S = shore, D = locked door, @ = undersea dwarf explorers
I've just finished the overhead magma channel and I'm about to start casting the obsidian in the ocean. In order to build this I found an ocean with a volcano on it. The volcano is extremely tall, many Z levels above the ocean, so I've built a very long suspended lavaduct high over the ocean to drop magma from a very great height. I don't have any bauxite for mechanisms/floodgates, so I can't stop the magma once I start the pump. I'll probably screw it all up once I start pouring, unfortunately...