Picture a world where land is practically nonexistent. The ocean stretches from horizon to horizon as far as the eye can see, and time is measured by the position of the sun and stars. All of life takes place on or above the water. This is Earth-31.
Citizens of this earth let industry and pollution get out of hand. They were unable to prevent global warming from taking hold and rising temperatures caused problems around the globe. Natural disasters wreaked havoc; continent-level tectonic events and mega-tsunamis sundered the land. Survivors of the cataclysm found themselves left on a post-apocalyptic world covered in water.
Though they must face myriad challenges to stay alive on the drowned surface, Earth-31's residents are not unprotected. Super heroes sail the Seven Seas and do all they can to maintain peace and stop piracy. Captain Leatherwing, Robin Redblade and the crew of the Flying Fox fly their flag high and bring safety wherever the wind may carry them.
BATMAN: THE DARK KNIGHT RETURNS #1 (1986)