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.