A map to which NO ONE HAS AN ANSWER-Magnetic rock in the CENTER?

The story of Hyperborea is thousands of years old, but what particularly attracts attention are the maps from the end of the 16th century that present the North Pole surrounded by 4 islands and a black rock in the middle. What is this really about and is it possible for Mercator to draw a map of the world that looks quite similar to the modern one including Antarctica and for the Arctic to be completely imaginary? Anyone who has a need and can help the work of the channel can do so via this link. Thanks in advance !