Deception Island is a volcanic island with a flooded caldera that has a complex geological setting in Bransfield Strait, Antarctica. We use P-wave arrivals recorded on land and seafloor seismometers from airgun shots within the caldera and around the island to invert for the P-wave velocity structure along two orthogonal profiles.  The results reveal a low-velocity anomaly beneath the caldera with a maximum anomaly of ~-1 km/s extending from the seafloor to ~5 km depth.  Refracted arrivals suggest a  >1-km-thick layer of sediments and unconsolidated lavas infilling the caldera.   Synthetic inversions show that this layer accounts for only a small portion of the velocity anomaly, implying that there is a significant region of low velocities at greater depths. Further synthetic inversions and melt fraction calculations suggest that the caldera is underlain by an extensive region of magma that extends downwards from <2 km beneath the seafloor.