Aestival

"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.

Oh How I've Missed It

Surface sunlight streams through, bathing the island in its glow and triggering intense rushes of pleasure. It's beautiful, but deadly to denizens of the Neath, who die on contact with sunlight due to the Sun reminding them of the laws of life and death. Unlike when visiting the Surface, where zailors will try to shield themselves from the burning sky, many crewmen who arrive here will happily give themselves over to the sunlight, going out with a final smile.

These factors, as to be expected, make Aestival one of the most dangerous places in the Neath. As a result, it is completely uninhabited, save for a few animals and dead bodies. It does make for a good place to harvest fruits and other bounties, but one has to be careful.

According to the Irrepressible Cannoneer, a meteorite-like Seal of the Red Science is what broke the roof.

A Colony? A Colony!

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? You'll need help from a certain Machine though.