Deep in western Russia, if you know where to look, you'll find a small collection of ragged scrap metal and crumbled concrete. Which isn't that exciting.

But if you rifle through the rubble, you will find a large metal disc bolted to the ground. This isn't just any old disc - it's the welded-shut cap of a borehole that plummets more than 12 kilometres (7.5 miles) into the Earth.

How deep is 12 kilometres, comparatively? Deeper than the deepest point of the ocean - the Mariana Trench. In fact, this is the deepest hole we've ever dug into our planet.

It's called the Kola Superdeep Borehole, and for once, it has nothing to do with extracting fossil fuels. The borehole exists purely for the wonderful science of it all.

When Soviet scientists started drilling down into Earth's surface during the 1970s, they did it to find out more about the contents of the planet's crust.

"Because the truth is, we know less about what's under our feet than what's on the other side of the Solar System," explains Hank Green in this 2014 episode of SciShow.

Over the next 24 years, these scientists kept at it on and off; while they didn't get down as far as they were hoping, by 1994 they'd made it just over 12 kilometres.

Which is certainly nothing to sniff at, because it happens to be a record-breaking dig that still stands today. The drill technology they had to develop to get down there is pretty remarkable.

But what did we actually learn from all this hard work? As Green explains in his video, we learnt a whole lot!

Firstly, the fact that there's water at 12 kilometres into Earth's crust, which scientists wouldn't have even thought possible had they not seen it with their own eyes.

And almost 7 kilometres down (4.4 miles), they found microscopic fossils of 24 species of long-dead single-celled organisms.

They also gained access to rocks 2.7 billion years old, which is awesome - but these rocks became the challenge that the scientists just could not overcome.

Why? Because their temperature was around 180 degrees Celsius (356 degrees Fahrenheit), about 80 hotter than what the scientists predicted, and they couldn't get past this heat.

Will we ever figure out how to get down further than this? Well, 1994 was a long time ago, so never say never.

A version of this article was first published in March 2015.