The Blue Machine

The easiest way to reach the mantle must be via the shortest hole, and that meant digging through thin oceanic crust rather than thick continental crust. This not only had to be the deepest hole ever dug, but it had to start in a place no human could touch. The entrance to the bowels of the planet would be on the deep sea floor. The name of the boundary between the crust and the mantle is the Moho, 13 and so, inevitably, the vertical tunnel that crossed that boundary would be the Mohole.