The ocean is the last physical frontier on Earth. Covering 70 percent of the planetary surface, it is the largest, most complex biome we know. The ocean is a huge, mobile reservoir of heat and chemical mass. As such, it is the “engine” that drives weather-climate systems across the ocean basins and the continents, directly affecting food production, drought, and flooding on land. Water is effectively opaque to electromagnetic radiation, so the seafloor has not been as well mapped as the surfaces of Mars and Venus, and although the spatial relationships within the ocean basins are well understood to a first order, the long- and short-term temporal variations and the complexities of ocean dynamics are poorly understood. The ultimate repository of human waste, the ocean has absorbed nearly half of the fossil carbon released since 1800. The ocean basins are a source of hazards: earthquakes, tsunamis, and giant storms. These events are episodic, powerful, often highly mobile, and frequently unpredictable. Because the ocean basins are a vast, but finite, repository of living and non-living resources, we turn to them for food, energy, and the many minerals necessary to sustain a broad range of human lifestyles. Many scientists believe that underwater volcanoes were the crucible in which early life began on Earth and perhaps on other planets. The oceans connect all continents; they are owned by no one, yet they belong to all of us by virtue of their mobile nature. The oceans may be viewed as the common heritage of humankind, the responsibility and life support of us all.