[quote=easter]So I guess my question is: should I consider the Neath essentially a different dimension? Like… hollow earth style.[/quote]There is a vague lore concept known as the Treachery of Maps that handwaves this.
[quote=easter]The canal is the ONLY known way in or out (besides the hole that shines on Aestival) and until the Bazaar pulls down a city, the surface and the neath are entirely independent of one another.[/quote]Not quite true. There is the Travertine Spiral that leads to the Surface. In the event of an apocalypse-level event in the Neath (say, the LIberation of Night), this will be the path most surviving civilians take to exit.
Furthermore, the Surface is aware of the existence of the Neath, though given that the only major interactions comes in the form of zailors and traders, it’s only natural that Surface-dwellers are skeptical of the tall tales of the Neath. Living devils from Hell? Walking squid creatures? Really? The nations of the Surface also have their own schemes; there’s a hint that some Surface nation is trying to mint currency out of Nevercold Brass, and there are occasional visitors/spies here to learn about the Neath.
[quote=easter]The elder continent is below Africa/southern Europe somewhere but also has the neath roof over it.[/quote]Attempting to define the location of the Elder Continent relative to the rest of Earth is a prospect of mere speculation and is relatively insignificant. I, for instance, would find it poetically significant to have the Elder Continent, as the site of the Garden, be located beneath Africa, the origin of Homo sapiens, but there is no hard evidence, as far as I know, to suggest any relative location.
Also, note that if London were to return to the Surface, it would likely destroy Naples when it resurfaces.
[quote=easter]The citizens of the neath, while perhaps (definitely in the Khanate’s case) descended from residents of the other cities, have existed independent of the surface for time immemorial. There is essentially an entire second world below the surface that doesn’t follow the surfaces rules.[/quote]Population figures are difficult to ascertain, but most residents of the previous cities suffered a more terrible fate. The Khanate successfully managed to break off from the Fourth City and the influence of the Masters, but most weren’t so lucky. The Second City was crushed before its people could execute their escape plan. Survivors of the Third City can be found in the Tomb-Colonies. I can only think of three confirmed survivors from the First City.
Many of the local settlements in Sunless Sea appear to have been settled by former Londoners. The origins of the rest are something of a mystery: Varchas closely resembles Angkor Wat but barring that the Hindu and Khmer influences are undeniable; Irem resembles Iram of the Pillars; and, of course, the Elder Continent human population seems to be entirely separate from those of the Surface.
[quote=easter]Additionally, I’m assuming the surface basically follows our rules. The whole "talking animals" and "sphinxes made of whispering rock" bits are neath thing that were as weird to Londoners when they first showed up as anything else? Like… the surface wasn’t some equally weird and esoteric thing before the Bazaar. Surface monkeys were just monkeys, surface cats were just cats.[/quote]Yep, though there are a couple notable differences. While the usual methods don’t work, there have been instances where Surface-dwellers have entered Parabola. A lot of the weirdness in the Neath is its own, but some came from the outside. The Correspondence hypothetically should still work on the Surface, and the eye-stealing sorrow-spiders hypothetically should function.