Flexi Says: Beneath the crust is the mantle. Earth’s crust is a thin, brittle outer shell. This layer is thinner under the oceans and much thicker in mountain ranges. The hot magma rises up into the rift valley at the mid-ocean ridges. The lava cools to form new seafloor. The youngest seafloor is at the mid ocean ridge that is the thinnest part of crust and is the best place of drilling to get to the mantle.