"Sun! A great beam of sunlight bathes the island! Squint into the dazzle: far above, there's a hole in the roof. In that light you see beaches of white-gold sand; trees heavy with bright fruit; the reds and blues and greens of the Surface."Aestival is an island in the Unterzee, above which lies a hole in the roof of the Neath. Surface sunlight streams through. It's beautiful, but deadly to denizens of the Neath, who die on contact with sunlight unless they haven't spent enough time down there to break the Judgements' laws.
It is possible to protect oneself and one's crew from the sunlight and found a colony there. One might just have to spend a lot of money to obtain the proper protection, because, well, sunscreen isn't a thing in the Neath, is it?