The last undiscovered frontier on Earth are the oceans. It is the vast body of salt water that covers almost three quarters of the planet’s surface. We know more about the surface of the Moon than we know about the ocean floor here on Earth. Complete darkness, freezing temperatures and crushing pressures are the formidable factors that make deep ocean exploration an extremely challenging task. The properties of salinity and temperature are investigated for different oceans around the world, including a look at the diversity of marine life and Coral Reefs. Early ocean voyages of explorers including the Vikings, Columbus and Magellan are described.