The deep oceans are one of the final frontiers on our planet. Scientific study is making rapid inroads into understanding processes and possibilities of the deep sea, but the technological challenges are enormous. This volume presents overviews of some of the fields of deep ocean science which will determine the course of exploration in the next decades, from studies of the deep biosphere living in the rocks of the seafloor, the quest to establish permanent observatories at many kilometers depth to the scientific background to the drive for extracting minerals from the seabed.