The asthenosphere is not the bottom part of the mantle. It's actually a layer within the upper mantle.

Here's a breakdown:

* Earth's Mantle: This is the thickest layer, sandwiched between the crust and the core.

* Upper Mantle: The top portion of the mantle.

* Asthenosphere: A soft, partially molten layer within the upper mantle. It's characterized by its ability to flow slowly over geological timescales.

* Lower Mantle: The dense, solid portion of the mantle below the asthenosphere, extending down to the core.

So, while both the asthenosphere and the lower mantle are part of the Earth's mantle, the asthenosphere is located within the upper mantle, not at the bottom.