In 1492, Columbus sailed the Atlantic Ocean．Up to that time, men from central Europe did not dare to sail west across the sea．Sailors were held back because they feared many ers．Their maps showed the unexplored parts of the world as erous and frightening.
In those days, the men who made maps did not know what the world was really like．They used their imagination．They showed the earth as a large circle with Europe and Asia in the center．Around the edges of the circle, they showed swamps(沼泽), deserts, and the ocean．One-eyed men and terrible monsters(怪兽)were supposed to live there.
The ocean itself was called the Sea of Darkness．Sailors believed that ff they sailed far north they would reach mountains made of ice．They thought that far south the ocean turned into fire and boiling water．They were afraid to sail east or west．They thought they would fall off the earth.
A sailor had to face some ers that were quite real．At that time, ships were not made with metal as they are today．Shipworms could attack a boat's wooden planking(船壳板)．A sailing ship might look strong but eaten through．It might go to pieces at sea and sink.
Men who made the early maps showed that the center of the earth as _______.