That would be the inner core because when it's under the crust, there would be one layer above it the outer, then the mantle way underneath since the Earth's crust is a thin shell on the outside of the Earth, accounting for less than 1% of Earth's volume. It is the top component of lithosphere : a division of Earth's layers that includes the crust and the upper part of the mantle. And the Earth's mantle is a layer of silicate rock between the crust and the outer core. Its mass of 4.01 × 10 24 kg is 67% the mass of the Earth. It has a thickness of 2,900 kilometres (1,800 mi) making up about 84% of Earth's volume. It is predominantly solid but in geological time it behaves as a viscous fluid.