A mirage is a real optical phenomenon (not a hallucination) produced by the refraction of light rays passing through air layers of different temperature and therefore different density and refractive index. When a strong thermal gradient exists near the surface, light rays curve gradually, creating displaced, inverted, or distorted images of distant objects. The inferior mirage is most common: it occurs when sun-heated ground creates a layer of warm, low-density air near the surface, beneath cooler air above. Light rays travelling downward curve upward, creating an inverted image that simulates a pool of water or a "lake" on the road. The brain interprets this inverted sky image as a reflective water surface. It is typical of deserts, paved roads in summer, and large sun-baked plains. The superior mirage occurs when a temperature inversion places cold air beneath warm air, typically over cold water surfaces or ice. Rays curve downward, making distant objects appear elevated or floating above the horizon (the Fata Morgana effect). This type can create multiple, distorted images of coastlines, ships, or islands. Superior mirages are occasionally observed over cold seas and polar regions, and have historically been responsible for phantom island sightings and other navigational mysteries.