The Marsili Seamount is the largest European underwater volcano. It is Plio-Pleistocenic in age, rising up to more than 3000m from the seafloor in the SE Tyrrhenian basin (Central Mediterranean), a back arc basin which began progressively opening 10 Ma ago (Kastens et al., 1988). The seamount lies in a key area for understanding the evolution of the Tyrrhenian region, characterized by high values of heat flow (Della Vedova et al., 2001) and low values of Moho isobaths (Locardi and Nicolich, 1988). In spite of the large dimensions of the Marsili seamount, we still have limited knowledge of its present activity. Ocean exploration is dependent on available technology and infrastructure, which started to develop strongly only after the 1980s. In fact, from its discovery in the 1920s, very little was known of the Marsili Seamount until the late 1990s when new techniques such as multibeam acoustic bathymetry were developed allowed to reveal at least the morphology. Some dedicated expeditions then obtained the first morpho-bathimetric map of the entire Tyrrhenian seafloor, based on multibeam swath-mapping together with seismic, gravimetric and magnetometric data (e.g. Marani and Gamberi, 2004). Although these data have greatly contributed to our understanding, the necessarily short measurement time limits the extent to which they reflect short- to medium-term geophysical processes in the Tyrrhenian basin. New technologies, such as multiparameter seafloor observatories, provide long-term continuous time-series in deep ocean waters, which are the basis for an original approach in ocean exploration. The observation of phenomena variability over time is key to understanding many Earth processes, among which we recall hydrothermal activity, active tectonics, and ecosystem life cycles. The development in Europe of multidisciplinary seafloor observatories has been pioneered under the EC Framework Programmes, specifically in the GEOSTAR projects (Beranzoli et al., 1988, 2000). From 2003 to 2005, long-term geophysical and oceanographic monitoring was conducted within the EC ORION-GEOSTAR3 project with two multiparameter observatories deployed on the seafloor 3320m below sea level (b.s.l.) in the vicinity of the Marsili Seamount. The two observatories were equipped with a set of sensors providing long-term continuous time-series of various physical measurements. The acquired time series are the longest continuous data record of the Marsili Basin available so far. This chaper intends to provide the main information on this experiment and present some results of the processing of the corresponding time-series, adding new valuable information on the still poorly explored activity of the volcano seamount. This chapter is organized as follows: The next section will provide the geological setting to understanding the importance of the Marsili Seamount and its basin; the ORION-GEOSTAR3 experiment is described in Section 24.3; some results from this unprecedented seismic, magnetic and gravimetric data analyses are shown in Section 24.4; and finally, in the last section we present our discussion with the main conclusions.Published623-6413A. Geofisica marina e osservazioni multiparametriche a fondo mar
