Ok, it is time for 'Unsolved Mysteries' with Doomdark as your host.
The Arabs, while perhaps erstwhile explorers in their own right, might have inherited some of their impressive geographical knowledge from other cultures. It has been shown that they possessed incredibly detailed knowledge of the world's geography.
Case in point: Admiral Piri Re'is map from 1513. He was an Ottoman Turk, but shared the Arab cultural heritage.
Piri Re'is map clearly depicts parts of the North American coast, most of the South American Atlantic coast line, the Andean mountains, the Amazon river and parts of Antarctica.
Piri Re'is himself noted that he had used Columbus's maps as references, and also some other (very enigmatic) sources.
More on Piri Re'is map.
...and a Skeptic.
[This message has been edited by Doomdark (edited 02-09-2000).]