The still largely unexplored vastness of the ocean, covering about 2/3 of the surface of the earth and with an average depth of almost 4 kilometres, has fascinated humans for as long as we have records. Its currents, chemical composition, and ecosystems are all highly variable at different locations and times. Due to its large capacity for storing and transporting heat, the ocean represents also a crucial component of the physical climate system. The ocean provides invaluable resources to humankind in the form of food, but also in terms of its generally moderating impact on weather and climate. This has led to the fact that a large fraction of the world's population lives within 100 miles of the ocean. As a consequence, the ocean and particularly the fragile interface between the ocean and the land has become subject to large environmental pressures.