The oceanic crust is part of the crust that is made fully out of basaltic rock and is several times thinner than the continental crust which is made out of granitic rock. Oceanic crust underlies Earth's oceans and is produced at divergent plate boundaries which are commonly referred to as mid-ocean ridges. As it grows from infilling magma and spreads away from the ridges, it will becomes cooler, thicker, and more dense, and will eventually sink into the mantle.
