Since the progress of things at Mark Reads has now taken us well past the point where this material is spoilery, I thought people might like to see a copy of my working map for Deep Wizardry, from which the map appearing in the original book was derived.
Information about this area was hard to come by in the early 1980s, and it was just as well that I was then often working on the book out of the Frederick Lewis Allen (writers’) room in the main branch of the New York Public Library, as its stacks were one of the few places where it was possible to lay hands on the research data from the Woods Hole Oceanographic Institute that was then laying bare the geography of the area around the Sohm Abyssal Plain. No existing map combined all the data I needed, so I wound up drawing this myself and then later adapting it in a slightly different format for the hardcover.
The full-sized map is here if you want to look at it in more detail.