Across much of the planet, temperatures increase by 25-30C (77-86F) on average every kilometre you go down through the Earth's crust. In the UK, for example, the subsurface temperature at around 5km (3 miles) down is about 140C (284F), according to the British Geological Survey.

Drill down far enough, though and it is possible to reach a point where water temperatures surpass 374C (705F) at pressures above 220 bars (one bar being average pressure on the Earth's surface at sea level). This is where water enters an energy-intense state known as supercritical, where it exists in a form that is neither liquid or gas. The hotter and more pressurised it is, the greater energy it contains.