>>713284938
There's no official answer to that (abyssal lore is very vague aside from some of them being sunken ships) but one of the trailers for KC Arcade shows an abyssal sitting at the bottom of the ocean looking all lonely iirc so maybe the abyss is more of an empty wasteland rather than a military base.
I do like to think that they do have some bases but the distance between them is long, with most of the abyss being a very dangerous place filled with deep sea creatures the abyssal girls either tame or hunt to use as equipment.