This volume is one of the most significant results of the conference "Science-Technology Synergy for Research in Marine Environment: Challenges for the XXI Century" held in Erice and Ustica, Italy, September 1999. It presents state of the art developments in technology and scientific research in sea floor observatories. Scientific conclusions of earth science and environmental studies obtained from these observatories as well as results from long term monitoring are provided. Descriptions of new technologies enabling deep sea long term observatories are offered and marine environment and risk assessment issues are discussed. This is the first work detailing recent and on going experiments world wide specifically devoted to deep sea multi disciplinary observation systems, the technology enabling sea floor observatories, and the presentation of first results from these systems.