The periphery of this map is composed of the watersheds of Asia, Europe and Africa. Each basin being attached at the ocean, sea or bay into which it drains. Inland basins, such as the Caspian, are attached at their lowest overflow points, as it were, their spouts. Deserts, such as the Sahara, follow the same logic. The threadlike white line seen in the Americas is the continental divide; it forms the edge of the Style “N” map (following this post).
The large red arrowhead at the map’s bottom, just a bit left of center, is where — so they say, if there are such things — great waves occur; the Indian Ocean surface current trying its best to squeeze its way around the southern tip of Africa and getting pinched now and then by the three currents to the south of it going the other direction.
world maps with constant-scale natural boundaries 