The French Ministere de la Recherche is funding a multidisciplinary project, the Polynesian Lithosphere and Upper Mantle Experiment (PLUME), to image the upper mantle structures beneath French Polynesia. This region of the southwestern Pacific, which is far from any plate boundary comprises oceanic lithosphere with ages varying between 30 and 100 Ma, as well as two major fracture zones. The area is characterized by a “swarm” of volcanic island chains—the Society Islands, the Austral Islands, and the Marquesas—that may represent “hot spot” tracks [Duncan and McDougall, 1976]. The individual hot spots are superimposed on the large South Pacific Superswell [McNutt, 1998]. The region is also characterized by a large-scale, low-velocity anomaly in the lower-most mantle [Su et al., 1994] and anomalous converted phases from the 660-km discontinuity [Vinnik et al., 1997].