The Isle of Mam was a phantom island, said to lie some distance west of the English Channel, approximately where the Great Sole Bank can be found. It should not be confused with the Isle of Man.

Although the Great Sole Bank is permanently underwater, it has been suggested that it may at one time have been visible, perhaps only at low tide, and it remained on maps for some time after it was no longer visible. Alternatively, the island was sometimes known as Brazil, and so might represent the same island as the Brazil off the west coast of Ireland.