The central part of the Earth's core, extending from a depth of about 5,100 km to the center (6,371 km) of the Earth; its radius is about one-third of the whole core. The inner core is probably solid, as evidenced by the observation of S waves that are propagated in it, and because compressional waves travel noticeably faster through it than through the outer core. Density ranges from 10.5 to 15.5 g/cm₃. It is equivalent to the G layer. Compare: outer core