A man taking a stroll along the coastline in Cornwall, England was "stunned" when he suddenly noticed a ship hovering high above the water. Of course it wasn't really floating through the air, but was the result of a rare optical illusion called a superior mirage.
From The Guardian:
The effect is an example of an optical illusion known as a superior mirage. Such illusions are reasonably common in the Arctic but can also happen in UK winters when the atmospheric conditions are right, though they are very rare.
The illusion is caused by a meteorological phenomenon called a temperature inversion. Normally, the air temperature drops with increasing altitude, making mountaintops colder than the foothills. But in a temperature inversion, warm air sits on top of a band of colder air, playing havoc with our visual perception. The inversion in Cornwall was caused by chilly air lying over the relatively cold sea with warmer air above.
Because cold air is denser than warm air, it has a higher refractive index. In the case of the "hovering ship", this means light rays coming from the ship are bent downwards as it passes through the colder air, to observers on the shoreline. Having evolved to keep things simple, the human brain is easily fooled. It assumes the light rays from the ship have travelled in a straight line, and so pictures the ship in a higher position than it really is – in this instance, above the sea surface.