There are ancient examples of the legged triskelion as well, from Greek art, for instance, while such "rotary" symbols more generally go back far into the millennia BCE, commonly with three or four "arms".

I fear someone will have suggested this already, but will there be a seal with a seal on? Or even a walrus? ?

@grandula - There is a specific Marine Maps style available through the 2017 Cartographer's Annual which, although a non-pictorial modern style, might be a place to start.

Some of us have been hoping for some underwater mapping options for years for CC3+ though, so you're not alone in this as an area of interest ?️!

I keep missing days here somehow; other distractions elsewhere. So apologies if you've already covered this, and I just missed it somewhere earlier, but should there be an accessway into area 31? There seems to be a corridor linking to the level, without a doorway or secret door marker.

If you've a suitable image you can and want to use (importing an entire spiral galaxy will likely crash your computer ?), and it's available to you as a standard image file (jpg, png, bmp), you can simply create a new Sheet in your CC3+ drawing, and then import it into that using the Draw - Insert File command from the drop-down menus. Give the Sheet a recognisable name so you can find it easily when you need to turn it on and off. You may need to resize the image to get it to fit the size of your drawing properly, but that's fairly easily done, if a bit fiddly to get exactly right.

You would probably be best to also set the file image up on a separate Layer too, but that's not essential.

If you do go down the "shadowed line" route, you'll probably need to fiddle about with a mask for part of the pit too, to stop the shadow going into places outside it you don't want - sadly, I remembered that only about ten minutes after I came offline yesterday...

Also - just seen this now - there's a gap at the SE corner of room 16. That may have happened when you cut the wall to insert the open doorway if you started the wall at the corner of the room originally. I learnt from one of Joe Sweeney's videos early on to always start a dungeon wall along one of the straights you won't be cutting a doorway into later, to avoid exactly this kind of issue. It's also easily forgotten when you're busy drawing walls, however, as I've discovered ?.