Ocean exploration is a part of oceanography describing more generally the exploration of ocean surfaces. It is also the period when people explored the ocean boundaries. Notable explorers include: the Greeks, the Egyptians, the Polynesians, the Phoenicians, Phytheas, Herodotus, the Vikings, The Portuguese, and Christopher Columbus. Travel on the surface of the ocean through the use of boats dates back to prehistoric times, but only in modern times has extensive underwater exploration become possible. Scientific investigations began with such early scientists as James Cook, Charles Darwin and Edmund Halley. Ocean exploration itself coincided with the developments in shipbuilding, diving, navigation, depth measurement, exploration and cartography.