There is an odd dichotomy in human exploration: While we think nothing of going up -- jetting through the skies six miles up, skydiving from the edge of space, or launching humans hundreds or thousands of miles into deep space -- going down has always proven rather difficult. To this day, the deepest humankind has ever gone is just 7.6 miles below our feet. It's not that we don't want to go deeper but, try as we might, despite millennia of developing advanced tools and materials, and exploration that has taken spacecraft to the edge of the Solar System, the subterranean depths remain firmly off-limits. Why?