I remember reading about a pre-2000 Japanese tabletop RPG with black-and-white illustrations, that's about dungeon-building, or being really focused on dungeons.
I think it has a nice tutorial section inside for how to play a person's first RPG, but what really stuck out was that its world was covered in dungeons, that were split between the land, air, and sea, with the oceanic zone just being called the The Depths/The Deeps.