In a couple of months, I'll be running a TTRPG intended as a crossover between Bioshock and the Chthulu mythos (plus a third, but I won't mention it here lest my players see it). The basic premise is that, at the dawn of the 20th century, R'lyeh rises, and alien forces rain down from the skies, rendering the surface uninhabitable. Humanity flees to a couple dozen bottle cities scattered beneath the oceans, as well as maybe a dozen more deep underground. Several hundred years pass; these cities lose contact with each other, and their inhabitants forget that they're not alone, or that there was ever any other way of life.

The players are going to be denizens of a London-sized underwater sprawl, a set of massive enclosed domes built from a combination of early-1900's tech and eldritch machinery. I haven't designed anything like this before, so are there any suggestions or resources for such a project?