The Earth comprises of a series of layers, that is, the crust, the mantle, the outer core, and the inner core.
The crust floats on a layer of semi-molten rocks known as the mantle. The mantle is the thickest layer of Earth, that is, of about 3000 kilometers and comprises of semi-molten rocks. In the mantle, the molten rock is known as magma.
Thus, mantle is the thickest layer of the Earth.