The existing oceans are geologically relatively young. They formed when a large supercontinent (Pangaea) broke apart some 150 Ma ago. The mid-oceanic ridges are narrow belts of submarine mountains marking the boundaries between plates which are moving apart, with new oceanic crust being created at the same time. Under the continents oceanic crust is subducted. The ocean bottom is an ideal location to study recent geodynamic processes on Earth. Of special interest for the reader will be the combination of the most recent oceanographic data with ophiolites, representing fossil ocean crust, obducted onto the continents by tectonic processes. The book is mainly written for students, teachers and scientists in the broad field of geosciences. In addition, it will be of interest to a broad readership since the text is accompanied by figures, most in color, of excellent quality and significance.