The thickest layer of the Earth is the mantle, the Earth is divided into four main layers: the crust, mantle, outer core, and inner core. The Earth mantle is about 1,802 miles thick, making up 84% of Earth's total volume.
The mantle is the most solid bulk layer of the Earth's interior because it is found between the core (the layer that have the highest temperature) and the crust (superficial layer), being the form of that layer between rock solid (upper mantle) to almost liquid (lower mantle), since the temperature, pressure, and molecules (iron and niclel) are why the layer is solid bulk.