We’re fascinated by the subterranean world at BBC Future. There’s something so compelling about the idea of an undiscovered and unseen place, just beneath our feet.
This week, we featured two stories from the underground – we wrote about the potentially deadly World War II bombs that still lie unexploded beneath cities, and discovered that hundreds of millions of barrels of oil sit unused in salt caverns, hoarded there so that the US has an insurance policy against global disaster.
Yet there are many more things to discover about the territory beneath us, as the following facts from the BBC Future archive and elsewhere show:
1. The walls in the TauTona gold mine in South Africa can reach 60C – at 3,900m below the surface, it’s the deepest human beings have ventured themselves.
Discover more about what you’d encounter if you could travel to the centre of the planet: Journey to the centre of the Earth – an interactive infographic
2. The monster machines that recently finished tunnelling a new transport network beneath London are so big that they have their own toilets and kitchens.
(Credit: Getty Images)
3. The deepest hole dug by human hand – without machinery – is the Woodingdean Well near Brighton in the UK. Reaching 390m (1,285ft) underground, it is as deep as the Empire State Building is tall, yet little more than a metre wide.
Read more: The deepest hole dug by hand
4. The contents of sewers can reveal the extent of illegal drugs taken within a city. Ecstasy use, for example, is particularly high in London.
Read more: The hidden drugs under your feet
5. Up to two million people in Beijing live underground – and many architects are dreaming up futuristic ways that you could too.
Read more: Will we ever live underground?
6. A vast network of tunnels lie beneath Liverpool in the UK – and the strange thing is that no-one knows why they were built.
Read more: The lost tunnels buried deep beneath the UK
7. There may be the equivalent of ‘oceans’ of water lurking inside planet Earth. We know this thanks to the chance discovery of a diamond containing a lump of ringwoodite that had been spewed out of a volcanic vent millions of years ago – it held 10 times as much water as previously thought.
Read more: Are there oceans hiding inside Earth?
(Science Photo Library)
8. Sometimes, the ground suddenly swallows entire homes – these sinkholes are caused when groundwater erodes a cavity beneath the surface. Fortunately, Nasa may have a way to see them coming.
Read more: Sinkholes: Can we forecast a catastrophic collapse?
9. When a nuclear bomb is tested, it sends shockwaves all the way through the Earth. These seismological traces are now used to find out if any nations are performing secret tests. (Bonus fact: the mass of the Earth is 5.9 sextillion tonnes: that's 59 followed by 20 zeroes.)
Read more from BBC Earth: What is at the centre of the Earth?
10. The foundations of the Shard skyscraper in London go down 53 metres in the clay beneath the surface, until they reach a layer of stiff sand. This is deeper than most skyscrapers in New York – the foundations of the Empire State Building are only 16 metres deep.
Read more from iWonder: How do skyscrapers stay up?
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