Question

Which part of the mantle is still a solid but flows like a thick, heavy liquid?

Answer 1

The asthenosphere is the hot, weak, inner part of the upper mantle. Although it is solid, it can flow like a thick, heavy liquid

The asthenosphere is the highly viscous, mechanically weak, and ductile region of the upper mantle of Earth. It lies below the lithosphere, between approximately 80 and 200 km below the surface. The lithosphere–asthenosphere boundary is usually referred to as the LAB.