The conquest of the abyss - the deep sea floor - is an exciting new adventure closely comparable with the space exploration program, but facing a more severe and extreme environment. This book addresses the important and apparently simple question 'How can continuous and reliable monitoring at the seafloor by means of seafloor observatories extend exploration and improve knowledge of our planet?' It will lead the reader through i) the present scientific challenges to be addressed with seafloor observatories; ii) the technical solutions for their architecture; iii) an excursus on world-wide ongoing projects and programmes; and finally iv) some relevant scientific multidisciplinary results. A presentation of new and interesting long-term perspectives for the upcoming years will complete the volume. Seafloor observatory scientific results will yield significant improvements and exert strong impact not only on our present knowledge but also on human life.