Goldenflame Dragon decided to humour us here at the Ultima Codex:

Ok, because the Ultima Codex asked for it, here’s a teaser for an early part of the Undersea Grotto. No monsters or loot yet, but there will be some.

You can see the screenshot above. In a comment to the above post, Goldenflame added some additional details about the dungeon walls:

Those walls were a huge PITA, let me tell you. I made them for a cave tunnel between two sides of a mountain range (which was its own challenge, as maps by default are only connected to one location on the surface, so this one did weird things with my version of the X-it spell, for instance), and to make them I actually made each possible combination of corners and edges and each tile of wall looks at what’s near it and figures out what it is bordering and picks the right graphic for that. I am not sure I am exaggerating if I say that fully half of the graphic files in the game are versions of this wall. There are better ways but I was lazy and brute forced it.

Having dabbled in tileset creation myself, I don’t envy Goldenflame the task of putting the graphics for the Undersea Grotto together. And in the end, brute force or not, it does look rather awesome, doesn’t it?