The original of this post appeared in 2012...
The children of port cities are wont to crowd the docks when any airship comes in, but none generate the excitement that the return of a vessel laden with strange, subnebulous treasures does. Many’s the young lad or lass who dreams of one day being one of the daring divers who brave weird miasmata and battle strange creatures to win fortune and fame.
The modern world has four strata. The highest is the upper atmosphere of relatively benign flying things. Just beneath are the High-Lands of plateaus and mountain-sides where humanity dwells. Lapping at these lands at the lowest elevations is the Fog Sea, a region of roiling, glowing, multicolored mists. These mists are eldritch things: toxic, mutagenic, or both, with lengthy and concentrated exposure. Inhospitable though this region may be to humans, there are many flying or floating creatures which make it their home.
The deepest depths of the fog shroud the lowest strata: the Low-Lands, the Undersea. Here one may find true oceans of water (gray and toxic from absorbing the overhanging fog), but more importantly, here lie the ruins of a once great civilization. This is thought to be the ancient home of man, before whatever happened, happened, forcing him to seek higher ground. Ancient treasures--both of wealth and knowledge--were left in these ruins. Though sailing a whole vessel through the fog is generally considered too risky a move, divers and diving craft are sent down to reclaim these treasures.
The fog isn’t the only danger. If the strange flying and floating things weren’t enough, the ruined cities themselves are inhabited by monsters. Some are mutated animals, others are humanoids--perhaps the degenerate descendants of the humans left behind. These savages view divers as violators of their territory at best and potential meals at worse. In the shadowy depths, divers do battle with these creatures, steel against steel, as firearms often misfire dangerously when submerged in the fog. The psychoactive properties of the mists have given strange powers to the creatures that dwell in it--but sometimes limited exposure does the same for divers, too.
Despite the dangers of death or loss of humanity, the rewards are great. There is no shortage of youths willing to sign on for a voyage beneath the Fog Sea.