Cross section through a class M world,featuring the orange coloured mantle
The mantle is the largest shell layer found inside a planet. On class M planet's the mantle is composed of melted rock, magma, which moves in convection currents and supports thin layer of solid rock, the crust, which may be habitable. (ST reference: Star Charts)
On the planetoid Darien 224 when an ancient Vulcan psionic weapon, the Vorl-tak, was fired it caused an energy feedback which shattered the planetoid’s mantle which began to sink into the magma layer. (EV comic: "Cloak and Dagger")