Pillars of Herakles. Greeks in the eighth century (and perhaps even earlier) conceived the earth to be a flat disk, on which the landmass of Europe, Asia, and Africa was surrounded by a vast river called Ocean. Thus, the inhabited earth was thought to be an "island" encircled by water. In the eighth century B.C.E. Homer and Hesiod used this image to good effect in their poetry; early Greek maps also showed this characterization. Although the water encircling the inhabited world was described as a "river," the ancient poets do not say what the boundary of Ocean was: early Ocean just stretches away into the measureless distance. The Greeks knew that the Strait of Gibraltar (the ancients called this spot the Pillars of Herakles) was to the west, and for them it marked the boundary between the known and the...