The asthenosphere is found beneath the lithosphere, which is the rigid outer layer of the Earth that includes the crust and the uppermost part of the mantle.

Here's a breakdown of Earth's layers and where the asthenosphere fits in:

* Crust: The outermost layer, made of solid rock.

* Lithosphere: The rigid layer that includes the crust and the uppermost part of the mantle.

* Asthenosphere: A semi-molten layer below the lithosphere. It's like a very viscous fluid.

* Mesosphere: The solid layer below the asthenosphere.

* Outer Core: A liquid layer composed mostly of iron and nickel.

* Inner Core: A solid, very dense ball of mostly iron and nickel.

The asthenosphere is crucial for plate tectonics because the lithospheric plates "float" and move on top of it. The asthenosphere's semi-molten nature allows for this movement.