Magnetic images of the region surrounding the Great Sumatra earthquake and tsunami of 26 December 2004 provide a current and historical view of subduction in the region. A quasi-static far-field image (Figure 1a) clearly shows the compressional boundary between the Eurasian and Indo-Australian plates. Along the fault rupture, magnetic crustal thicknesses increase markedly in an easterly and northeasterly direction. Both the island arc and the subducting slab are magnetic, and the subducting slab is diving into the mantle at a steep angle, further increasing the magnetic thickness. To the east, between Singapore and the south coast of the island of Borneo, a previously unrecognized first-order feature parallels the currently active subduction zone. Like the present subduction zone, the feature is characterized by a two-fold to three-fold increase in magnetic thickness in a northeasterly direction. While the feature's origin is enigmatic, it probably reflects the past history of subduction in the region.