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Description: Earthquakes at Campi Flegrei have been low-magnitude and sparse for more than thirty years, denying onshore monitoring observations of their usual source for structural constraint: seismic tomography. Here, we used ambient seismic noise recorded between 2011 and 2013 to reconstruct period-dependent Rayleigh-wave velocity maps of caldera-wide structures and volcanic reservoirs. The lowest velocities have been aseismic since 1985 and correspond to a fluid-storage zone that was fractured during the 1983-1984 volcanic unrest. Earthquake locations show that fluids migrate from the reservoir towards the Solfatara and Pisciarelli fumaroles along shallower low-velocity fractures. The Neapolitan Yellow Tuff rim faults bound high-velocity intra-crater domes, a product of historical eruptions, which act as a barrier for deep fluid migrations. The structurally-controlled reservoir is likely the shallowest product of a deep-seated offshore source SE of it, causing bradyseism and heating the caldera. The spatial correlations with regional ongoing dynamics and observations from historical unrests mark the reservoir as the most likely feeder pathway for fluid and magmatic inputs from this source.