* Crust: The Earth's outermost layer is the crust, which is the least dense.

* Mantle: The mantle is a thick layer of mostly solid rock, but it behaves like a very viscous fluid over long periods. It's denser than the crust due to the increased pressure and the presence of heavier elements.

* Outer Core: This liquid layer is even denser than the mantle, primarily composed of iron and nickel.

* Inner Core: The Earth's innermost layer, the inner core, is the densest. It's solid iron and nickel, incredibly compressed due to the immense pressure.

So, while the mantle isn't the least dense layer, it's also not the most dense.