That portion of the Earth extending from the lower boundary of the mantle to the centre of the Earth. The radius of the core is about 3500 km; the core's surface is about 2800 km below the Earth's surface. Only longitudinal (P) seismic waves will propagate through the core; trans-verse (S) seismic waves will not. Because liquids will transmit longitudinal but not transverse seismic waves, the core (or at least that part adjoining the mantle) is thought to be liquid. There is some seismological evidence that a region from the centre of the Earth out to about 1400 km is partly or entirely solid. This portion is called the inner core; the other, liquid portion is then called the outer core.