The deepest hole humans have ever drilled into the Earth is just 12.3 kilometers, about 0.2% of the distance to the planet's core. That effort, the Soviet-era Kola Superdeep Borehole, took more than 20 years to complete. And despite decades of technological progress since, we haven't gone any deeper. So what's stopping us? Business Insider visited drilling sites and spoke with engineers and industry experts to understand what it would take to go deeper than ever before, and whether reaching t...