Nauru Island, in the central Pacific Ocean, is a raised atoll capping a volcanic seamount rising from an ocean floor depth of 4300 m. The land area is 22 km2, and the island rises to 70 m above sea level. Drilling has proved dolomitized limestone of Late Miocene or younger age to a depth of 55 m below sea level. Gravity and magnetic surveys indicate that the substructure of Nauru is approximately radially symmetrical. Bouguer anomaly increases by 18 mgal from the coast to the centre of the island. The magnetic field has a range of 830 nT with a negative anomaly located over the north‐northwest coastline and a large east‐west‐elongated positive anomaly over the southern sector of Nauru. Modelling of the magnetic field suggests that the island is underlain by a reversely magnetized volcanic core with magnetization 1.5–1.9 A/m and depth of 500 m. The orientation of the magnetization vector gives the palaeomagnetic age of the core as mid‐Eocene to Oligocene. This age range for the evolution of Nauru is confirmed by an estimate based on island subsidence. Nauru may have been constructed by volcanism associated with Cainozoic transit across the Samoan hot spot. Gravity modelling indicates a density of 2.5 t/m3for the island pedestal. The calculated values for density and magnetization are typical for Pacific seamounts, implying that basement beneath Nauru's carbonate platform is composed mainly of basaltic lavas.