Speakers of many languages around the world rely on body‐based contrasts (e.g., left/right) for spatial communication and cognition. Speakers of Yupno, a language of Papua New Guinea's mountainous interior, rely instead on an environment‐based uphill/downhill contrast. Body‐based contrasts are as easy to use indoors as outdoors, but environment‐based contrasts may not be. Do Yupno speakers still use uphill/downhill contrasts indoors and, if so, how? We report three studies on spatial communication within the Yupno house. Even in this flat world, uphill/downhill contrasts are pervasive. However, the terms are not used according to the slopes beyond the house's walls, as reported in other groups. Instead, the house is treated as a microworld, with a “conceptual topography” that is strikingly reminiscent of the physical topography of the Yupno valley. The phenomenon illustrates some of the distinctive properties of environment‐based reference systems, as well as the universal power and plasticity of spatial contrasts.