An unexplained set of arrivals has been observed on ocean bottom seismometers (OBSs) during the NPAL04 long-range ocean acoustic propagation experiment in the North Pacific. The observed intensity pattern of the OBS arrivals is significantly more complex than the waterborne arrivals seen on the deep vertical line array (DVLA). These "deep seafloor" arrivals occur later than the first PE predicted arrival; their arrival time is not predicted by acoustic PE propagation models, they do not correspond to decay from shallower turning points (as is the case for deep shadow zone arrivals), and they are not readily observed on the DVLA hydrophone just 750 m above the seafloor. The arrival structure in the observed data, in time and amplitude, varies substantially between three OBSs that are separated by less than 4 km. Could these unexplained arrivals be scattering or horizontal multi-path from persistent ocean thermal structure?.