The thickest layer of the atmosphere is the thermosphere. It is located above the mesosphere and below the exosphere, extending from about 80 to 1,000 kilometers (50 to 620 miles) above Earth's surface.

While the thermosphere is the thickest in terms of its vertical extent, it is important to note that it's also the least dense layer. This is because the air molecules are extremely spread out in the thermosphere due to the very low atmospheric pressure.