Horizon is a vast pocket of air. A perfectly symmetrical cube with 1.200.000-plethron long edges, according to the finest measurements of the Mengzi Astronomical Institute. 

Horizon is a Moebius cube. Each face of the cube is the planar lower half-space of its opposing face. If one travels on a straight line long enough, they will finish their journey arriving at their point of departure, only this time, they will be coming from the opposite direction.