We present a comprehensive review of seismic and gravity observations and tomographic models produced over the past four decades in order to understand the structure of the crust beneath the Campi Flegrei caldera. We describe the main lithological and structural discontinuities defined through these observations, illustrate their geophysical responses, and discuss the constraints they give to the understanding of magmatic and volcanic processes. Micro-seismic crises related to caldera unrest, and ambient seismic noise measurements provide comprehensive seismic data to local earthquake and ambient noise tomography. In combination with reflection data from onshore and offshore active seismic experiments, velocity tomography reconstructs the elastic properties of the caldera between surface and ~4 km depth. Active experiments also define the depth of lithological interfaces and deep (~7.5 km) partially molten bodies. Seismic attenuation tomography provides information complementary to velocity tomography, defining lateral lithological changes and the geometry of onshore and offshore fluid and magma bodies down to 4 km depth. Once compared with seismic analyses, gravity data highlight lateral changes in the offshore caldera structures. During the deformation and seismo-geochemical unrest (1982–1984), they permitted to reconstruct a minor (<1 km lateral extent) melt volume related to the point of maximum uplift measured at the caldera. Seismic coda-wave amplitude inversions depict the caldera rim limits in analogy to velocity tomography and map the lateral extension of ~4-km-deep deformation source. Once combined with the results from velocity tomography and gravity inversions, they reconstruct the feeding systems that connect deep deformation source and shallow vents across the eastern caldera, capped by a seismic horizon around a depth of 2 km.
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