Part 2/9:

Despite its vast potential, geothermal energy has critical limitations. The primary challenge is drilling deep enough to access the geothermal heat stored roughly 10 kilometers or more beneath the Earth's surface. Historical attempts, such as the German Continental Deep Drilling Program and Russia's Kola Superdeep Borehole, reached 9.1 km and 12 km, respectively, but not without tremendous costs and time – 24 years and about $1 billion for Kola.