Also known as hafgerdingar effect. Hafgerdingar is Norse for 'sea hedges' or 'sea fences'. The term denotes a circular *physical illusion depicting a giant wave similar to those occasionally reported in Polar sea regions. The term hafgerdingar stems from a 13th-century manuscript called the King's Mirror, which contains descriptions of Iceland, Ireland, and Greenland. For a long time it was believedthatinthismedievaltexttheterm hafgerdingar refers to a circular tidal wave or a single rogue wave occurring at open sea, caused by a submarine earthquake or a capsizing iceberg, and actually capable of putting a ship in grave peril. However, a re-examination of the hafgerdingar's original description indicates that the phenomenon in question may well have been a * superior mirage or *fata morgana. In the Polar region such physical illusions can appear to the observer as a huge wall of waves surrounding one's ship, with an apparent height of 30 m or more.
References
Lehn, W.H., Schroeder, I.I. (2003). Hafgerdingar: A mystery from the King's Mirror explained. Polar Record, 39,211-217.
Dictionary of Hallucinations. J.D. Blom. 2010.