Earth's core is a scary place, when you think about it. If you travelled straight down for around 2,900km (1,802 miles) – that's roughly equivalent to travelling from London to Edinburgh four or five times over – you'd reach it. Specifically, you'd have dug through Earth's mantle to reach the outer core. Here, rock gives way to liquid metals and the temperature soars to nearly 2,700°C (5,000°F). Dig (or swim?) a little deeper, another 2,000km (1,243 miles) or so, and you'll reach the inner core. Here, temperatures double, rising to over 6,000°C (10,800°F) – as hot as the surface of the Sun. At this temperature almost everything vaporises, even diamonds, but the material here is under enough pressure to keep it liquid.
Most of this heat is residual, left over…
