A compass termite mound is a thin blade of hardened earth, often more than three metres tall but only a fraction of that thick. Across a colony field the blades run close to the magnetic north-south axis, so consistently that early surveyors used them for bearings.
The geometry is thermal. At dawn and dusk, when the air is cool, the sun strikes a broad face and warms the mound. At midday, when it is overhead and brutal, only the narrow ridge is exposed. The result is a mound that heats early, stays workable through the day, and avoids the worst of the tropical noon. Field measurements have shown a real temperature gradient through the structure that the termites exploit by moving between chambers.
Experiments with artificially altered magnetic fields indicate the termites orient using magnetic cues rather than only the sun's path, though how they detect the field is not settled. The habitat matters: these plains flood in the wet season, so a tall mound also keeps the colony above water, and the two pressures, heat and flooding, are hard to separate.