Step 1: Understand that the Earth has different layers: the crust, mantle, outer core, and inner core.
Step 2: Know the approximate thickness of each layer: the crust is about 5-70 km, the mantle is about 2900 km, the outer core is about 2200 km, and the inner core is about 1270 km.
Step 3: Compare the thicknesses of the layers to find out which one is the thickest.
Step 4: Conclude that the mantle, at about 2900 km, is the thickest layer of the Earth.
