Everyone knows the Earth's core is hot, but maybe the scale of it still has the power to surprise. Temperatures in the iron center of the core are estimated to be around 5,200 °C (9,392 °F), generated by heat from radioactive elements decaying combining with heat that still remains from the very formation of the planet – an event of cataclysmic violence when a swirling cloud of gas and dust was crushed into a ball by its own gravity.
Where there's access to heat, there's harvestable geothermal energy. And there's so much heat below the Earth's surface, according to Paul Woskov, a senior fusion research engineer at MIT, that tapping just 0.1 percent of it could supply the entire world's energy needs for more than 20 million years.
The problem is access. Where subterranean heat sources naturally occur close to the surface, easily accessible and close enough to a relevant power grid for economically viable transmission, geothermal becomes a rare example of totally reliable, round-the-clock green power generation. The Sun stops shining, the wind stops blowing, but the rock's always hot. Of course, these conditions are fairly rare, and as a result, geothermal currently supplies only around 0.3 percent of global energy consumption.
If we could drill deep enough, we could put geothermal power stations just about anywhere we wanted them. But that's harder than it sounds. The Earth's crust varies in thickness between about 5-75 km (3-47 miles), with the thinnest parts tending to be way out in the deep ocean.