Here's why:

* Crust: This is the outermost layer, composed of relatively light materials like granite and basalt. It's the thinnest layer, with an average thickness of about 30 kilometers (19 miles).

* Mantle: This is the thickest layer, made up of denser rock materials.

* Outer Core: This layer is composed mostly of liquid iron and nickel, making it significantly denser than the crust and mantle.

* Inner Core: This is the Earth's solid, innermost layer. It's composed of mostly iron and nickel and is the densest layer due to immense pressure.

Therefore, the crust, being the layer composed of the least dense materials, is the least dense layer of the Earth.