A vast network of 200-year-old tunnels lies beneath Liverpool’s streets – and no one knows why they’re there. Chris Baraniuk meets the dedicated volunteers digging up their secrets.

The air is still. It’s quiet. Occasionally, the sound of a water droplet bursting feebly onto stone echoes through the chamber. Somewhere, somehow, moisture is getting in. But for the most part, it’s dry. And were it not for the smattering of electric lights, this 200-year-old tunnel beneath the streets of Liverpool would be very dark – and very lonely.

Best of 2015

Our top stories

This is part of BBC Future’s “Best of 2015” list, our greatest hits of the year, including:

How dark is your personality?
The dystopian lake filled by the world’s tech lust
How to spot a liar
The downsides of being drop dead gorgeous

…and much more. Browse the full list.

“I still can’t get over the ferns and the moss,” says Dave Bridson, a local historian and manager at the Williamson Tunnels heritage centre in Liverpool, England. He points out where the water seeps through the porous stone, nurturing the light green moss that has formed spontaneously next to lightbulbs. Ever since light was brought into the long-lost tunnels, little pockets of vegetation like this have taken hold.

It took years, however, for that light to arrive.

Of all the engineering projects that ever took place in the industrial centre of Liverpool – like the world’s first exclusively steam-powered passenger railway – the building of the Williamson Tunnels in the early 19th Century must be the most mysterious. The patron of the tunnels, tobacco merchant Joseph Williamson, was extraordinarily secretive about their purpose. Even today, no one is sure exactly what they were used for. Nor does anyone know for sure even how many of the tunnels there are, scattered underfoot beneath the Edge Hill district of Liverpool in northwest England.

Meanwhile, for centuries, the tunnels had been buried. They were filled in after locals complained of the smell – apparently the caverns were long used as underground landfills and stuffed with everything from household junk to human waste.

For centuries, the tunnels had been buried. They passed from knowledge to myth

As time went by, the tunnels passed from knowledge to myth.

“A lot of people knew about the tunnels, but that was as far as it went – they just knew about them or heard about them,” explains Les Coe, an early member of the Friends of Williamson Tunnels (FoWT). “It was just left at that. But we decided to look for them.”

Breaking in

On a summer day in 2001, Coe and a small band of investigators literally “broke into” a suspected tunnel in the Paddington area of Edge Hill. With the help of a digger, they made a small hole in the roof of what turned out to be an old cellar: the upper level of one of the tunnel systems.

Coe and a few others gingerly ventured in via a harness. The chamber was full of rubble piled so high, walking upright was impossible. Still, the explorers were thrilled. “It was quite exhilarating when we found that opening,” Coe recalls.

Eventually, three different sites in the area would offer access to various bits of the tunnels. But excavating them was – and still is – difficult work. Over the last 15 years teams of volunteers, digging up to twice a week, have removed more than 120 skips of waste material. They have revealed forgotten cellar systems and, in several cases, multiple levels of tunnels – some with stone steps leading down to deeper caverns. There are also some debris-filled passages branching off in odd directions; it’s not clear how far they go or to where they ultimately lead.

Volunteers have removed more than 120 skips of waste material

Tom Stapledon, a retired television engineer and shopkeeper, is one of the regular excavators. He explains how early tests with metal rods, plunged down into the coke-like rubble had revealed the unexpected depth of the chambers. “They put a 10ft (3m) rod in and they didn't hit the bottom. Then they put a 15ft (4.6m) rod down and didn't hit the bottom,” he says. Only a 20ft (6m) rod eventually struck the solid floor – at 19 feet (5.8m) down.

The excavation work is not easy. Besides the sheer manpower needed, the volunteers have to get special permission from the council to dig in any new direction. Sometimes, it’s not granted for safety reasons.

“There are flats and things above us. We can’t go digging too much,” says Bridson with a laugh as he points out one partially opened channel that leads to another rubble-filled cranny.

They’ve uncovered ink wells, poison bottles, chamber pots and hundreds of clay pipes

Stapledon, though, has his eyes on one blocked-up tunnel that runs under a street. The team suspects that it could lead to a whole other system of underground chambers still undiscovered.

As they excavate, the volunteers methodically document any artefacts they find. So far, they’ve uncovered ink wells once used by schoolchildren, bottles that held everything from beer to poison, jam jars, ceramics from Liverpool’s Royal Infirmary, oyster shells, chamber pots, animal bones and hundreds of clay pipes – a tapestry of household bric-a-brac that tells social history of Liverpool over the last two centuries in a way no other collection can.

“It’s a history lesson,” says Stapledon as he points out a favourite find, a cup commemorating the coronation of Edward VII in 1902. He holds it up to the light and an image of King Edward VII himself appears in the base of the mug, cleverly embossed into the ceramic. “Absolutely beautiful,” he says in wonder. “I don’t think we’ve come across another one like it.”

The king of the hill

Why the tunnels are even here might be another history lesson – but, for the most part, is a mystery.
Born in England in 1769, Joseph Williamson was a successful tobacco merchant. He ploughed his money back into the local area – Edge Hill – by employing men to build houses. After the Napoleonic Wars, unemployment was rife across Britain; Williamson presumably saw a philanthropic opportunity to engage them in the area’s development. It was perhaps thanks to this that he got the nickname ‘The King of Edge Hill’.

He also had men build tunnels. One entrance to the system even has been found in the basement of his former house. But why tunnels? Did he ask them to build his tunnels arbitrarily, for no other purpose than to be paid for work? It seems extraordinary. And yet there are no known records from Williamson’s time which offer anything like an explanation for their construction.

There are no known records from the time which offer anything like an explanation for the tunnels’ construction

Instead, succeeding generations and historians have had to guess – leading to all manner of speculation. Perhaps Williamson wanted secret passages to get to and from buildings in Edge Hill. Or was a smuggler and needed the tunnels to carry out covert operations.

Or maybe he and his wife belonged to a fanatical religious cult that anticipated the end of the world, and his tunnels were designed to provide shelter during the apocalypse. Apparently, someone once made the suggestion casually on television, and the idea since stuck.

Not with Bridson, however. “Total guff,” he says, chuckling. “He was quite a devout Christian and a Church of England member.”

Perhaps Williamson’s tunnels were designed to provide shelter during the apocalypse

Those who have worked on the tunnels have now developed a new, somewhat more satisfying theory. Bridson points out a series of markings in the sandstone that he says are indicative of quarrying. There are channels to drain rainwater away from the rock while men worked, blocks out of which sandstone could be hewn, and various niches in the walls where rigs were once likely installed to help with extracting the stone, commonly used as a building material.

Bridson believes that before Williamson came along, these pits in the ground already existed. But it was Williamson’s idea to construct arches over them and seal them in. Properties could then be built on top of the reclaimed land – which otherwise would have been practically worthless.

If this was the case, then in terms of land reclamation, Williamson was way ahead of his time, says Bridson. The work may well have hastened the development of an area that, without this innovation, would have been left unused for many years.

Williamson also was enterprising in his design. Simply filling the trenches in would have taken too long in the early 1800s, thanks to the limitations of transport, so Williamson used arches instead. And as Bridson notes, he was doing it years before the great railway tunnels and bridges of England were ever built. The arches “are still standing 200 years on with virtually no maintenance,” he says. “Apart from the ones that have been damaged, they're still as solid as the day he built them. So he must have known what he was doing.”

The arches are still as solid as the day he built them. He must have known what he was doing

For now, the quarry reclamation theory remains just a theory. Bridson hopes that one day he’ll find a stash of letters and documents in Williamson’s handwriting that once and for all settles the argument. “Part of me clings on to that hope,” he says. But he admits it’s not likely such a trove will ever be found.

Mystery motivation

Perhaps that’s no bad thing. Tom Stapledon says that the volunteers often debate whether they’d like to find Williamson’s papers at all. If they were never discovered, the mystery of what lies waiting to be found can be allowed to linger, motivating the diligent few who work on the excavations week in, week out. That mystery is what drives them on.

The Williamson Tunnel excavators are almost all retired men, Liverpudlians with enough time and curiosity to devote to the project. Younger men, says Stapledon, ask to volunteer now and again, but they usually move on after a few weeks. “They don’t have the stamina we do,” he jokes.

Even 200 years after Williamson offered ready work to the men of Edge Hill, his tunnels are still keeping the locals busy.

Now, it’s the end of a long day of digging; another skip has been filled to the brim with excavated dirt. The steel covering that protects one of the tunnel entrances has been tightly padlocked. Stapledon checks it. “Secure,” he says.

There’s little that would mark out the presence of the tunnels in this spot to passers-by. But they’re down there, right under the feet and homes of Edge Hill’s residents. And, it seems, Liverpool’s lost tunnels are finally giving up their secrets – bucket by bucket and inch by inch.

Follow BBC Future on Facebook, Twitter, Google+ and LinkedIn. This story is a part of BBC Britain – a new series focused on exploring this extraordinary island, one story at a time. Readers outside of the UK can see every BBC Britain story by heading to the Britain homepage; you also can see our latest stories by following us on Facebook and Twitter.