thumb|upright=1.3|Part of the of 1539 by [[Olaus Magnus, depicting the location of magnetic north vaguely conceived as "Insula Magnetu[m]" (Latin for "Island of Magnets") off modern-day Murmansk.]]
thumb|upright=1.3|Detail from [[Gerardus Mercator's map of the Arctic ( edition), showing the Rupes Nigra at the North Pole ('POLVS ARCTICVS'), surrounded by four large islands.]]
The Rupes Nigra ("Black Cliff"), a phantom island, was believed to be a black rock located at the North Magnetic Pole or at the geographic North Pole itself. Described by Gerardus Mercator as 33 French miles in size, it provided a supposed explanation for why all compasses point to this location. The idea came from a lost work titled Inventio Fortunata, and the island featured on maps from the sixteenth and seventeenth centuries, including those of Mercator and his successors. Mercator described the island in a 1577 letter to John Dee: