The Rupes Nigra ("Black Rock"), a phantom island, was believed to be a 33-mile-wide black rock located at the Magnetic North Pole or at the North Pole itself. It purportedly explained why all compasses point to this location. The idea came from a lost work titled Inventio Fortunata, and the island features on maps from the sixteenth and seventeenth centuries, including those of Gerardus Mercator and his successors. Mercator describes the island in a 1577 letter to John Dee:
- In the midst of the four countries is a Whirl-pool, into which there empty these four indrawing Seas which divide the North. And the water rushes round and descends into the Earth just as if one were pouring it through a filter funnel. It is four degrees wide on every side of the Pole, that is to say eight degrees altogether. Except that right under the Pole there lies a bare Rock in the midst of the Sea. Its circumference is almost 33 French miles, and it is all of magnetic Stone (...) This is word for word everything that I copied out of this author [Jacobus Cnoyen] years ago.