Short answers, pulled from the story.
Earth's mantle is a layer of silicate rock between the crust and the outer core. Its upper mantle is dominantly peridotite, composed of olivine, clinopyroxene, orthopyroxene, and an aluminous phase that shifts from plagioclase to spinel to garnet with depth. The lower mantle is composed mainly of bridgmanite and ferropericlase.
Earth's mantle has a thickness of 2,900 kilometers, about 46% of Earth's radius and 84% of its volume. It makes up 86% of the mass of Earth.
Temperatures in Earth's mantle range from about 500 kelvin at the boundary with the crust to about 4,200 kelvin at the core-mantle boundary. Despite far exceeding the surface melting point of peridotite near 1,500 kelvin, the mantle stays almost exclusively solid because enormous pressure raises the temperature at which melting begins.
Earth's mantle is predominantly solid but behaves as a viscous fluid on geologic time scales, sometimes described as having the consistency of caramel. The temperature difference between the surface and outer core drives slow, creeping convection, with hot material rising in mantle plumes and cooler, heavier material sinking at subduction zones.
Earth's mantle is divided into three major layers defined by sudden changes in seismic velocity: the upper mantle from the Moho down to 410 kilometers, the transition zone from about 410 to 660 kilometers, and the lower mantle from about 660 to 2,891 kilometers. The lowest roughly 200 kilometers form the anomalous D double-prime region.
Scientists explore Earth's mantle mostly at the seabed because oceanic crust is thinner than continental crust. Efforts include the abandoned Project Mohole in 1966, the Deep Sea Drilling Project from 1968 to 1983 using the Glomar Challenger, and JOIDES Resolution, which in 2023 recovered cores 1,268 meters deep from the Atlantis Massif consisting largely of peridotite.
Seismic images reveal two continent-sized anomalies in the lowermost mantle with low seismic velocities, which are denser and likely compositionally different from the surrounding mantle. They may represent buried relics of Theia mantle material left after the Moon-forming event proposed in the Giant-impact hypothesis.