Like the island of Hybrasil its substance appears and disappears depending on the prevailing psychology of the recipient.
Hybrasil was a mythical island located in the Atlantic off the West coast of Ireland. At the time of the last Ice Age, 8,000 years ago, sea levels were about 140 metres lower and it is thoguht that the elevated undersea here (Porcupine Bank) was above water. On maps it is usually depicted as a circle with two slashes across it - a valley or a mountain range?
A good website to visit for further information is here : http://historicmysteries.com/hy-brasil-the-other-atlantis/