Mt. Vesuvius is a small volcano associated with an elevated risk. Seismic data were used to better define its magmatic system. We found evidence of an extended (at least 400 square kilometers) low-velocity layer at about 8-kilometer depth. The inferred S-wave (~0.6 to 1.0 kilometer per second) and P-wave velocities (~2.0 kilometer per second) as well as other evidence indicate an extended sill with magma interspersed in a solid matrix.