TThe earth’ s Interior Planet Earth has three main parts: At the center is the densest part the core (metallic iron, nickel). Surrounding the core is the mantle Surrounding the mantle lies the thinnest and outermost layer the crustThe Earth’s Interior ◼ Planet Earth has three main parts: ◼ At the center is the densest part, the core (metallic iron, nickel). ◼ Surrounding the core is the mantle. ◼ Surrounding the mantle lies the thinnest and outermost layer, the crust