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How deep can we dig

For some reason, humans like to dig holes. We dig holes to bury our dead, to find shiny rocks, to make basements, to find Stanley Yelnats’ hidden treasure, the list goes on. But all of this digging, at least, has a rationale behind it – a purpose to justify the effort. And while today we do have some examples of those, we also have an example of a hole that was dug… for the hell of it, basically. Because today, we want to ask the same question that was asked fifty years ago when they started it: how deep can we actually dig?

The Art of Digging Holes

Of course, the answer to the question gets complicated when you try to get more specific. First off, how are we digging this hypothetical hole? That might seem like a trivial question; just grab a shovel and start throwing dirt. But digging holes by hand is actually incredibly hard, as anyone who’s had to do so can attest. To give you an idea, let’s talk about the Woodingdean water well, located in Woodingdean. Obviously.

Woodingdean water well

This well was dug in 1862, by hand, to a depth of 390 m (1,280 ft), and believe me when I say that it was miserable. It took a team of diggers four years of working in atrocious conditions to get this well finished, and one man actually died after falling to his death at the bottom of the shaft. The Woodingdean water well is the deepest hole in the world that was dug by hand, and compared to other holes we’ve dug since, it didn’t go very far at all. So… yeah. If we want to dig a really deep hole, doing so by hand is probably the worst option. No, this is a job for technology.

Nowadays, when people dig really deep holes with advanced industrial tools, it’s usually because it’s a mine. On that note, let us turn to the undisputed king of deep mines: South Africa. Of the 22 deepest mines in the world, 16 of them are in South Africa. All of those mines, by the way, are gold mines, proving that even though we don’t use gold coins as currency anymore, old habits still die hard. But let’s now turn to the deepest mine on the list, the Mponeng Gold Mine, which reaches 4 km (2.5 mi) into the earth, or just under five Burj Khalifas underground.

It is here that we start to see the effects of the primary obstacle to digging deep holes: the temperature. The Earth is a giant hot ball of rocks, and the deeper we go, the more clear that becomes. The temperature in the Mponeng mine can reach up to 66 degrees Celsius, or 151 degrees Fahrenheit. That’s… uninhabitable. The mine has to pump slurry ice underground in order to cool the air to something that won’t literally kill people working in it. And bear in mind, we haven’t gone very far at all: the Earth’s crust, or the outer layer of the planet, is anywhere from 30 to 50 km (20 to 30 mi) thick. And of course, the crust is the thinnest part of the planet, accounting for less than 1% of the total volume of the Earth.

Earth layers

There’s also another problem. Rocks start to behave strangely when their temperature increases past a certain point. In the mantle, the largest section of the Earth’s interior, the heat creates something called convection currents, where hotter rocks move towards the crust and cooler rocks move towards the core. It’s more like a sea of caramel than a solid layer of rocks, and… well, just imagine trying to drill through a thick layer of caramel. It’s messy. And delicious. But mostly messy.

There’s also the issue of the Earth’s core. The core is made of two parts – the outer core, and the inner core. In the outer core, the temperature sits at around 3-4 thousand degrees Celsius, or 5-7 thousand degrees Fahrenheit. If the rocks in the mantle are kind of goopy, here they’re just completely melted into liquids. Last we checked, you can’t really drill through a liquid.

And then there’s the inner core. Here, it’s still just as hot as the outer core, but the force of gravity is so strong that the materials within, despite being more than hot enough to melt into liquid, are forced into a solid state by the weight of the planet around them. The inner core is so dense that seismic waves actually reflect off of it during earthquakes, which incidentally was how we discovered the existence of the inner core in the first place. You’d need Unobtanium or something to get through that.

So, we’re not getting to the center of the earth. Not that you’d want to, anyway, you’d die – if not by the heat, then by the dinosaurs that still live down there. (The latter is only if you live in the Jules Verne novel.) But, can we at least get all the way through the solid bit: the crust? Well, we’ve listed off examples of holes dug for an actual reason, and now is the point we switch over to the other category we mentioned at the start: a hole dug for the hell of it.

Don’t Dig Straight Down

In May of 1970, the Soviet Union decided that they wanted to drill a hole straight through the entire Earth’s crust because, why not. In reality, the US held the record for the deepest hole and the Soviets wanted to beat it. So, they got a heavy drill, they found a spot, and started drilling. And unlike most Soviet engineering projects, this actually seemed go pretty well for a while. Well, it is just drilling a hole, so whatever.

This hole was named the Kola Superdeep Borehole, so named because it was drilled in the Kola Peninsula, it was super deep, and it was a borehole. This is the kind of intellectual breakdown you come here for, viewer. The Soviets drilled this hole over the course of two decades, reaching a final depth of 12,262 m (40,230 ft). And here, again, the primary obstacle was the temperature. At this depth, the rock was sitting at around 180 degrees Celsius (356 degrees Fahrenheit). The Soviets had expected it to be much cooler than that, but the fact that it wasn’t meant that the rocks were behaving “somewhat like plastic”. That’s… very bizarre to think about, even after we talked about the mantle.

Whatever the case, further drilling was considered unfeasible, and the hole was sealed up and abandoned. It’s only 23cm wide, by the way; you could trip on it if you weren’t paying attention. And for twenty or so years, that was that – we decided there wasn’t much point in going further. Until we did.

Kola Superdeep Borehole
Abandoned Kola Superdeep Borehole

We will now rapidly shift back to holes dug for an actual reason, but we’re not leaving Russia. We are, however, going to the exact opposite end of Russia. Just off the coast of the island of Sakhalin, are a set of oil and gas fields located underwater, deep in the Earth’s crust. In these fields, a joint project between the Sakhalin government, the Russian federal government and a subsidiary of ExxonMobil has been going since 2003 to tap the large petrol reserves in these fields. To that end, they have drilled a borehole even deeper underground than the Kola Superdeep Borehole, down to a depth of 15 kilometers as of late 2017. According to the rules of the game, then, capitalism is officially better than communism. I can’t wait to read the comments.

In conclusion, the answer to the question of “How deep can we dig?” is rather complicated. If it’s a simple answer you’re after, it’s probably around 30-50km, give or take, since that’s where the Earth’s crust officially turns into the Earth’s mantle, which as we’ve stated before is rock in name only. Even getting there would prove difficult, as the Kola borehole showed us, but even so, it’s not clear why we would do such a thing. Then again, that didn’t stop us before, so who knows at this point. Just don’t let the dinosaurs out.

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