Haiiro no Sora de Oote o Furuu

The year 23XX. Remembering what month and day it is today holds little meaning. The sun vanished due to a runaway AI, plunging this world into darkness and extreme cold. In this ruined world, a young man who had lost the meaning of life wandered into an unknown underground city. There, he found a bizarre city built from countless tangled pipes, and a vampire girl living in solitude. His encounter with the girl and his days in the city began to breathe life back into the young man's hollow heart. But soon he learns—the inescapable secret and fate that binds the vampires sealed within this underground city.