What should you do with the vast caverns left by Cheshire's salt mining? Fill them full of valuable documents of course.
During our lives, we all generate a vast paper trail of documents about us that need to be kept and archived. We rely on employers, doctors, councils, banks, the police and others to keep these details safe.
But - with so much paperwork - many filing cabinets and warehouses are full to bursting. So now many of our personal records are being transferred underground - to be stored down Britain's oldest working mine.
Because we're so deep underground, there's no UV light, there's no insect life and no vermin
Steve Holmes Deepstore
It looks like something out of a science fiction film. At the end of a long dark tunnel, deep beneath the Cheshire countryside, a mechanical monster groans into life.
It judders forward - its skin caked in clay, its menacing steel claws lunging into the rock face ahead.
A wall of salt millions of years old succumbs in seconds. It drops onto a conveyor belt where it's carried away to be crushed into tiny pieces.
It's all masterminded by remote control. Where once teams of miners spent a lifetime hacking at the rock with handpicks and shovels, now the same thing is achieved by three men and a keypad. This is what 21st Century mining looks like.
At 130 tonnes, it's thought to be the biggest underground machine in the world. There's no way it would fit in any of the lifts, so it had to be brought down in bits, and assembled where it stands.
The mine is thought to be the oldest still in operation in the UK
It's never seen daylight, and never will. Once its life is over, it will be left to be eaten by the salt. The earth will have its revenge in the end.
But for now the chopping and clattering goes on. The noise echoes down a man-made network of tunnels and caves carved out over decades.
This is the oldest working mine in Britain - it supplies about half of the salt used to grit the country's roads.
All this mining has left behind an awesome sight - a hollowed-out city the size of 700 football pitches, the roof propped up by vast pillars of rock salt that glisten almost pink under torch light.
But now this emptiness is being filled with millions of boxes. There are bank details, dental records, hospital X-rays - all paperwork that has to be kept, but which offices above ground are running out of shelf space to store.
So vast swathes of personal data are being transferred underground. Chances are something about your life is salted away here.
"Security is our top priority," says Mike McAuley as he takes me inside one of the specially-built storage rooms.
"Anybody up top needs to go through a scanning gate. They're checked by security guards. They're checked by CCTV cameras.
"If they were to get underground, they could only get inside the storage units with their own personalised access card and they're only issued to staff after in-depth police checks."
As soon as the door opens, the smell of salt is replaced by the smell of cardboard. Each unit houses thousands of boxes, neatly arranged on shelves that stretch to the ceiling.
Some contain irreplaceable historic artefacts. There are records from the National Archives listing the transport details of every convict sent to Australia in the 1800s. There are rare first editions from the Bodleian Library in Oxford. There are paintings, film cans, even a grand piano.
Over time, the seawater evaporated, leaving vast salty deposits that were gradually covered over. Some are 100m deep, others well over 1.6km - a mile - underground
Everything is barcoded so it can be retrieved within hours if needed.
"If I were an archive document, this is where I'd want to live," smiles Steve Holmes. He's in charge of Deepstore, the company that runs the facility.
"The natural properties of rock salt means the temperature underground is a constant 14C, and the humidity is around 60-65%, which is the ideal environment for the long-term storage of documents. Because we're so deep underground, there's no UV light, there's no insect life and no vermin."
Steve tells me there's no risk of flooding.
"We're protected by five layers of rock salt above us so there's no chance of water permeating through. Also, we've worked with industry experts to make sure there's no way there could be a fire."
Because the mine is still working, with 70 years worth of salt still in the ground, it will be a long time before all the storage space is filled. And despite the predictions of a paperless world, more boxes arrive every day.
While physical footprints fade, here our paper footprint survives.
It is a unique, accidental archive of the way we live - preserved for future generations.
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