The principal conclusion of this paper is that regional seismic data for deep and shallow earthquakes associated with the Tonga-Kermadec arc show that there exists in the mantle an anomalous zone whose thickness is of the order of 100 km and whose upper surface is approximately defined by the highly active seismic zone that dips to the west beneath the island arc and extends to depths of about 700 km. Limited data for other areas suggest that similar ‘anomalous zones’ are associated with other island arcs. The most important evidence for the above conclusion is that seismic body waves, particularly S waves, propagating in the anomalous zones are much less subject to attenuation than are waves of the same type propagating in parts of the mantle at similar depths elsewhere. One interpretation of the above results and of some additional seismic data is that the deep anomalous zone is continuous with the uppermost mantle east of Tonga. Such a structure is consistent with theories of mantle convection with down-going currents in the vicinity of island arcs. If low attenuation of seismic waves correlates with strength, this structure suggests that the lithosphere has been thrust or dragged down beneath the Tonga arc and hence implies a certain mobility for the lithosphere elsewhere. This possibility suggests, in turn, new approaches to a wide variety of problems ranging from the nature of the earthquake mechanism to the relation between complex surface geology and driving mechanisms whose configuration may be relatively simple.