For a million years the planet flooded, and for a million years man hid in an underwater city. In their giant submarine they would bode their time away from the storm and from the monsters lurking in the sky.
Ultranautica was the largest man-made structure in history. When democracy failed a totalitarian system was imposed so that every citizen would tend to a certain aspect of submarine life now that everyone was totally dependent on another for survival. The government adopted a strict eugenic policy so that one's descendants would be better at their job than they were. And so peace reigned in the final refuge of man.
Or so it was for the first few centuries. Nobody remembers what happened afterwards but a combination of factors resulted in the near-complete collapse of societal order in Ultranautica. Nevertheless many areas remained habitable. Civilization lived on albeit in increasingly primitive forms, each generation more ignorant than the last. Soon nobody remembered how the machinery functioned and why they worked to maintain it.
But they continued to toil nonetheless. Ultranautica twisted their flesh and bone. In her womb they became children. In the years to come civilizations rose and fell and man diverged into a panoply of grotesque shapes.
Explorers from the habitable zones would call them "abhumans" for the first few centuries, but eventually there came a point when a few species no longer resembled humans at all. Structural decay had created so many ecological niches that soon enough an ecosystem of uncanny monsters evolved from humans, shiprats, insects, and stranded aquatic life teemed in rusting jungles of machinery, waiting to be unleashed on an unsuspecting world.
At last the waters receded. Ultranautica balanced precariously on a tooth-like mountain overlooking a valley reeking of putrefying fish, buried in three feet of mud and silt. The doors, rusted shut, were blown open with gunpowder made from saltpeter gleaned from evaporated human urine, charcoal made from burned human bones, and sulfur obtained from human flesh.
And for the first time in a million years, Man walked the earth, changed.