Can electromagnetic techniques be employed to detect the presence of a plume below an ocean approximately 5 km deep? Recent magnetovariational data from the islands of Oahu and Hawaii provide compelling evidence that this question can be answered in the affirmative. A significant diminution of the vertical magnetic field is observed at Hawaii as compared with Oahu, which cannot arise from island/ocean bathymetry effects. The plume paradox is that the effect may not be explained by induction. This could imply distortion and current channelling due to mantle heterogeneities on a scale hitherto unconceded in global induction studies. ¿ 2000 American Geophysical Union