Next step: I've heard it's possible to alter the arena. If we can make it a huge cube of magma, do so. This will let us drop much larger, more complex diving bells into it, and hopefully let us design one where the dwarves stay on the inside.
I just tried something along these lines. Unfortunately, it seems that the arena's size is hard-coded at 144x144x9 -- so no matter what you put in arena.txt, you can't get an arena with more than 9 z-levels. This rather limits the height of things you can drop into the magma. At most, you can get four z-levels of magma below four usable z-levels of open space (since rock created on the top z-level will stick to the arena's ceiling).
To accomplish this, cover the interior of the bottom layer with smooth floors, then fill the next four layers with magma, and the rest with open space. That allows the bottom layer to fill with usable magma on startup. (If you just fill the bottom layer with magma in arena.txt, it'll all drain out immediately.) If you want to try it out and save some time, you can download my arena map here:
http://ii.fobby.net/df/arena_magmapit.zipI guess attempts at larger submarine will have to be carried out in fortress mode. (Unless we can hack DF to support a larger arena?) Anyway, I will continue experimenting with things. . . for SCIENCE.