On the long undersea plains of Rebma, there are chasms and crevasses and trenches. These gaps in the sea floor are particularly treacherous; they have become known as Deeps. The Deeps are cold, dark places, home to monsters.
In the Deeps, there is no trace of light from the surface, only the ghostly luminescence of sea creatures that hunt in the dark. The cold and pressure of the Deeps are so great that even native Rebmans risk their lives attempting to swim through them. The things that do live there are not sentient races for the most part, but are instead scavengers who seek out creatures that foolishly enter the Deeps, or predators who hunt the scavengers.
Regarding Depth Pressure
Pressure is a natural phenomenon that exists in all environments with atmosphere. It is compounded underwater by the density of the water, so that air-breathing humans feel increasing levels of discomfort starting at -30ft, to a maximum of about -100ft unassisted, and -300ft with normal scuba gear, and -1000ft taking extraordinary measures.
Water-breathing sentient races are admirably adapted to the environment, and can descend much deeper than humans before feeling that discomfort from atmospheric pressure. This range can start at about -3500ft and reach a maximum of -6500ft to -8500ft unassisted, depending on the race and individual.
The ill effects of atmospheric pressure to an otherwise healthy individual can be soft tissue damage, blood toxicity, and/or loss of consciousness.
Regarding Cold Exposure
The thematic upshot of these rough calculations is the creation of a forbidding and hostile zone in Rebma that not even the Sea's favoured individuals can go for more than a couple of minutes at a time. Even aquatic animals have species-specific limits to their depth survivability, although there are many creatures that inhabit the deepest of water by virtue of their specialized biology. These creatures comprise another type of lethal danger faced by those trying to visit the Deeps. For those who 'rise' to meet these formidable challenges, the rewards of discovery can be considerable.