Part I - Present scientific challenges to be addressed with seafloor observatories 1. Integrating continuous observatory data: A multidisciplinary view of the ocean in four dimensions,- 2. Underwater neutrino telescopes for astroparticle physics, 3. Seafloor observatories: the challenges for geo-hazards Part II - Technical solutions for seafloor observatory architecture 4. The role of Information Communication Technologies (ICT) for seafloor observatories: acquisition, archival, analysis, interoperability,- 5. Long-term sub-sea observatories: design, communications and deployment operations,- 6. Development and demonstration of a mobile response observatory prototype for sub-sea environmental monitoring: the case of ROSE,- 7. Design of seafloor observation system for high density low noise broadband observation,- 8. Implementation for seafloor observatory construction by ROV,- 9. Seafloor observatory architecture: the evolution from the past to the future Part III - World-wide recent and ongoing projects and programmes 10. The two seafloor geomagnetic observatories operating in the western Pacific,- 11. The DELOS project; long-term observatories in an oil field environment in the Tropical Atlantic Ocean off Angola, West Africa,- 12. Sub-sea environmental observatory integrated with the KM3NET neutrino telescope infrastructure in the Mediterranean Sea,- 13. ESONET NoE: An European initiative to develop and integrate the scientific European community using deep- sea observatories,- 14. ESONET NoE: An European initiative to develop and integrate the scientific European community using deep- sea observatories,- 15. ANTARES Neutrino telescope,-16. The NEPTUNE Canada Project: installing the world's first regional cabled ocean observatory,- 17. Active processes at mid-ocean ridge hydrothermal systems: a multidisciplinary observatory experiment at the Lucky Strike vent field, Mid-Atlantic Ridge,- 18. EMSO: the European Multidisciplinary Seafloor Observatory Part IV - Relevant scientific results with a multidisciplinary emphasis 19. Seafloor observatory for monitoring geological phenomena below ocean abyss,- 20. Modelling of regional geomagnetic field based on ground observation network including seafloor geomagnetic observatories,- 21. Geomagnetic observatories at the deep seafloor may enable early tsunami warning,- 22. Sub-sea borehole observatory over 8 years in Pacific plate subduction zone,- 23. An insight in the Marsili volcanic seamount (Tyrrhenian Sea), results from ORION experiment,- 24. Development and Application of advanced ocean floor network system for mega thrust earthquakes and tsunamis ,- 25. NEMO-SN1 cabled observatory after 3-year experiment: multidisciplinary results and future plans,- Conclusion -- looking forward to tomorrow's challenges.