The world is a Shoreless Sea and has been so for centuries now. Sky above and water below- there is nothing to see above your head, not even a sun, and the water stretches on forever into the depths. It is not merely water but pure water, the base elemental substance from which the Old World and its gods arose. It is Chaos.
Long ago, the world began to fall apart. Anyone that could be sure of the cause died long ago, and though there are many stories that have been passed down there is no way of verifying which, if any, is the correct tale. All of the evidence, like everything else, has disintegrated. All that one can be certain of now is that one day the world simply began to fall apart. Within a few short decades the world below had entirely dissolved back into Chaos. Refugees fled to the world of the firmament and the crystal spheres beyond (and some even reached these fabled safe havens) but the decay was not to be limited to the world of mortality, as it was then known. Just as quickly as the world below had fallen apart so too did the firmament and the crystal spheres.
And yet there is still life in this world after the world. Life aboard the husengels, flying craft with artificially-strengthened "integrity fields" to fight off the disintegrating influence of Chaos. So long as the field is maintained, the husengel will remain intact. Husengels are cities in themselves and depend on nothing save the very Chaos which they are fighting off. Formers are trained in manipulating their personal integrity fields in order to organize pure water into whatever is needed- fuel for the husengel, of course, but food, clothing, and other necessities as well. Even water needs to be reorganized, since pure water will damage your natural integrity field. Each husengel is a little world whose life totally depends on the organization of Chaotic matter.
This is where the primary danger comes in. Husengels must either descend to the Sea themselves or, if large enough, send gathering craft down in order to resupply their tanks. There are Intelligences in the Sea, masses of Chaos which have self-organized into cognizance. They typically disintegrate in short order but those that last long enough to make the jump to self-awareness can maintain themselves indefinitely and reach immense sizes as they continue to grow. They home in on other organized masses- like people- and there's no telling when a patch of pure water may actually be a lurking Intelligence until the waves beat against your craft in a lifelike manner and water shapes itself into teeth and spines and back again before the attack can be reacted to, only for tentacles to form and strike from another direction entirely- the only sure way to combat an Intelligence is to extend one's integrity field and shape the Intelligence into something else, but more than one former has extended herself too far and been manipulated in return. An Intelligence presented with such an opening might either possess her or just tear the field apart and reduce her to pure water in an instant.
The mental powers of an Intelligence grow exponentially with its size and especially dangerous Intelligences, those large enough to endanger entire husengels, are referred to as titans. Those that become large enough begin to manifest peculiar quirks of behavior and ability, and there have been numerous cases where a titan has organized a small world (really no more than a large island at best). These worlds inevitably fall apart as soon as it is assaulted by other Intelligences or its own creator loses interest, and the husengels of today are crewed exclusively by the descendants of those that found and still find these worlds to be too dangerous to consider settling. As a result the husengels universally avoid these worlds except to exile criminals that threaten to disrupt the order that exists on the ships (when even the smallest of malfunctions can kill the entire population if left unchecked for too long, husengel populations are also exclusively descended from the crews that were most fastidious and disciplined).
Intelligences are not the only danger to be had in the Deep. Chaos itself is a psychoreactive substance which can be influenced subtly by emotions and thoughts. Scared or angry gatherers can quickly turn the Sea around them into a roiling tempest, for which reason they are given drugs to dampen emotional extremes before heading out. On husengels, the tanks, too, can be dangerous if they are large enough, and similar precautions are taken around them. Without any sun there is only one natural source of heat: Chaos, which is never a very friendly source. When passive, there is only enough heat to keep the temperature just above freezing (and that's not counting the wind chill), and when reacting to minds and storming up the water's temperature quickly soars past the boiling point.