Twenty years ago the sky closed and the world stopped being warm. Nobody in this story explains what happened. Nobody who remembers is in a position to tell you, and the people who survived have more urgent problems than history. What's left of humanity lives in the canyons of the Ethiopian highlands, because a canyon holds heat and a plateau does not. At the centre of every sector is a machine nobody alive knows how to build. A vast ring, fixed into the rock, with a core that turns inside it and grinds horizontally into the stone. The friction is the heat. The heat is the sector. Teams work the winders in shifts to keep it moving, and the whole canyon hums at one low note, so constant that nobody hears it until it drops, and then every head in the city comes up at once. Heat is rationed to a household, not a person. Your name on the register is the only thing standing between you and a night outdoors. Nothing new is manufactured. Everything is salvaged, repaired, repaired again. O...