Light is a tricky thing in the underworld. Inhabitants from the surface find the underworld caverns too dark too see most of the time. Yet the glowing fungus covers most ceilings and magic that creates light is fairly common in most places. So true darkness is rare.
Yet, light is still fleeting. Even people whose ancestry has lived in the underworld for a hundred generations can’t handle true dark. They mock the people who live in the light of the inner sun… But they don’t forget that the underworld caves and tunnels have been civilized and made safe in comparison to what they were.
Flat surfaces to walk on. Fungus that sheds faint light. Animals that are strange and mutated, but at the core familiar to anyone from the surface. There are multiple levels but the caves are still mostly two-dimensional safety wrapped around the inner world of the endless sun. Dangerous but understandable.
Only the most desperate, crazy, or determined go outside of that small tamed underworld into the true darkness where light is everything and understanding is a rare luxury. In the Veins of the Earth there is only darkness and paranoia.
Still better than the Abyss, though.