"Doable" is very different from "economically justifiable". The thickness of the Earth's crust is strongly bimodal, making "average" meaningless: thinner under

"Doable" is very different from "economically justifiable". The thickness of the Earth's crust is strongly bimodal, making "average" meaningless: thinner under oceans, thicker on continents. Much thicker. Most of the planet is, of course, sea floor, so mostly thinner. But drilling the sea floor is unpleasant and expensive, and at the end, the hole is all the way down there.