What mechanical layer lies below the lithosphere

The layer below the lithosphere lies the Asthenosphere.

Explanation:

The asthenosphere is a viscous layer, in which the rocks are solid but has the ability to slightly flow due to deformation. This layer plays a very significant role in the plate tectonic movement, which initiates the motion of the lithospheric crust, due to the generation of the convection current at the mantle. It is present in the upper part of the mantle.

The internal heat from the core of the earth radiates to the mantle, and the magma at the mantle gets heated up and become less dense and rises up, and this magma when reaches on the upper part of the mantle, near the surface, gets cools down and become more dense, and sinks. This continuous circulation of magma in the mantle region is known as the convection current.

neither accurate nor precise.