The strong lower part of the mantle is a physical layer called the mesosphere.

Here's why:

* Mesosphere: This layer is characterized by its rigidity and strength. It's located below the asthenosphere (the weak, partially molten layer), extending to the core-mantle boundary. The mesosphere is solid but can deform under extreme pressure.

* Other layers: The mantle is also divided into the upper mantle and the lower mantle. While the upper mantle contains the asthenosphere, the entire lower mantle is considered the mesosphere.

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