The ocean characterises planet Earth and differentiates it from the more than 5000 known planets. It is also the habitat where life emerged as a singularity in the universe. However, we know less about the oceans than we do about some remote planets such as Mars. This book provides basic facts for understanding the oceans, their properties and their importance to mankind through the ages. The long relationship between the ocean and humanity, which permeates all areas of our culture, is at a crossroads where human activity is profoundly affecting its functioning and the integrity of marine ecosystems. However, the intelligent and rational use of the ocean holds, in large part, the key to providing the resources needed to sustain our quality of life in the future. Unlocking, through scientific research, the secrets that the ocean still holds is an essential challenge for humanity. This book informs the reader about all these aspects of the ocean through a text that combines scientific rigour with personal reflections based on the author's own experiences, dedicated to exploring and learning more about the ocean.