In the game of
Civilization, the discovery of Map Making allows you to build a
trireme, your first unit that can move over water. However, triremes suck. You can also build the
Lighthouse wonder, which gives you an extra movement point over water.
Map Making is tricky buisness, because you have to figure out a way to express the features of a sphere on a plane without introducing too much distortion. There's no satisfactory way to do this on a global scale.