Ask Glenn

Could you dig your way through the Earth to Australia?

Err... no, not really. Even if you could dig that deep, you’d be squashed flat or melted before you got there – by the searing temperatures and crushing pressures inside the planet.


Why’s it so hot inside the planet?

Because when the planet was still forming, about 4½ billion years ago, its building gravity drew in massive amounts of rock and ice, in the form of asteroids and comets. These fiery missiles bombarded the Earth non-stop for thousands of years, and the massive amount of energy release actually melted the planet. It has been cooling down ever since and, while the outside has cooled enough to form a hard crust, the inside is still mostly liquid, or semi-liquid (a bit like gooey, melted plastic).

So we live on the crusty bit, then?

Exactly. We, and everything else on the Earth, live on that thin shell or crust. This is less than six miles thick in some places (like under the deepest ocean trenches), but up to thirty miles thick in others (like under Mount Everest).

If it’s so thin, shouldn’t it be easy to dig straight through?

Well the best we’ve managed so far is a hole 12,262m (or about seven miles) deep, drilled over nineteen years from 1970–1989, on Russia’s Vola Peninsula near the Norwegian border. At this depth, the temperature was over 300°C, and the drill was damaged as it hit a pocket of molten sulphur. Plus, as the hole wasn’t started at the bottom of an ocean trench, this still wasn’t enough to make it right through the crust. So, no – it’s not that easy.

What if you could dig deeper – what would you find?

Beneath the crust is a layer of semi-molten rock called the mantle. This is about 1,600 miles thick, and makes up about two thirds of the Earth’s entire mass. It’s mostly made of metals like iron, aluminium, magnesium and silicon, all held in a gloopy, plastic state at temperatures of over 1,000°C.

Beneath that, there’s the core of the Earth. This is separated into an inner core and outer core. The liquid outer core is about 1,500 miles thick, and is mostly made of sulphur and iron. This has a temperature of over 3,700°C, and flows round the solid inner core. The inner core is made of iron, and is even hotter (over 4,300°C) than the outer core. But due to a weird effect called pressure freezing, it’s actually solid. So, if you made it down that deep, you’d have a job digging through that!

The Earth - Inner Core, Outer Core, Mantle and Crust

All right, but what if you could? Could you get right through to Australia on the other side?

OK, let’s say you dig through the solid iron core. You survive the burning temperatures and keep going – through the outer core, the mantle, and the finally the crust – to the other side. You still wouldn’t have made it to Australia.

Why not?!

Because Australia isn’t exactly opposite the UK on Earth.

What?! Well, what is, then?

Nothing, I’m afraid. Digging straight down from the UK, you’d pop out somewhere south of New Zealand, in the middle of the Antarctic Ocean. Probably quite annoyed with yourself.

That’s not fair.

Heh, heh. Sorry. If you check it out on a globe, most tunnels dug from Europe, the US or Africa would end up in the ocean. But here are a few trans-global tunnels you could try:

  • Chile to China
  • Spain to New Zealand
  • Brazil to the Philippines
  • Canada to Antarctica

Bah. Canada to Antarctica? Hardly seems worth the effort.

Unless you’re a polar bear. Plenty of tasty penguins down there, you know...