Tink I learned that because of the light refraction, you can sometimes even see things below the horizon! And the article does indeed give a simple, usable explanation... "The air is usually warmest near the surface of the sea and cooler above. Where there is an upwelling of cold water from the sea's depths, the air immediately above the water is cooled, creating a "thermal inversion" with warm air lying above cold air. Light is refracted as it passes through layers of air with different densities, and a thermal inversion may bend it into curved paths around the curvature of the Earth.
These can also look like castles; I learned about the term Fata Morgana, "from the Italian name for the fairy enchantress Morgan le Fay, is a complex, multi-layered superior mirage. Multiple images of the landscape are stretched, compressed or reflected and stacked on top of each other. Fata Morgana can turn low islands into towers, domes and columns, giving the illusion of fairy cities or phantom mountain ranges. In former times, the Fata Morgana could mislead navigators into believing they were close to land; the sailors of Messina believed it was a lure placed by evil spirits."