To constrain the upper depth range of lateral heterogeneities beneath Mt Etna, a seismic tomography method with artificial sources is designed to undershoot the target and restrict the depth of seismic propagation by reflection at the top of the basement. A strong high velocity anomaly thus imaged southeast of the summit, interpreted as a magmatic body, is reliably constrained to reach at least as shallow as 2 km depth where exsolution of SO2 from magma may occur. The high gas output rate of Etna [Allard, 1997> hence appears as the normal complement of the huge magma body seismically imaged. This magma body is located beneath the volcanic edifice rather than within, which attests to the control of volcanism by extension. These volatile output and internal structure distinguish Mt Etna volcanism among current types and may relate to the peculiar context of a recent regional change from subduction-related compression to extension. ¿ 1999 American Geophysical Union