An underground network of tunnels was discovered under Liverpool. Now workers think they've solved the mystery

CNN — Liverpool, England (CNN) — It's an unassuming entrance to what's been described as the world's largest underground folly, a centuries-old mystery buried at least 60 feet below the streets of Liverpool, England, and stretching out perhaps for a square mile or more.

The peeling, graffiti-covered facade of a Georgian house is all that remains at ground level, the front wall propped up by steel beams and the unglazed windows framing blue rectangles of sky.

A small plaque on the wall's left side declares that this is the former home of Joseph Williamson (1769-1840), "The Mole of Edge Hill," and that the property is in the care of The Friends of Williamson Tunnels.

Our Sunday afternoon tour group gathers at a construction trailer on what is a working excavation site, to put on hard hats and yellow high-vis jackets.

We're about to enter one of England's most unusual tourist attractions: a multi-level network of tunnels and chambers, dug on the instruction of tobacco merchant Joseph Williamson in the early 19th century, and built under his home and the surrounding neighborhood

Covering an estimated area of perhaps six acres or more, the tunnels were filled in with rubble and lost to the world for more than a century, before being rediscovered at the turn of the 21st century, and then painstakingly excavated.

The decades-long mission of volunteer diggers, including our tour guide Tom Stapledon, has been to answer one question: Why were they built? Wild theories have sprung up over the years, from them being used for the smuggling of contraband, to being the headquarters of an apocalyptic cult.

"We have struggled for 30 years with our research, trying to make sense of the story, and it's very difficult because he was a very secretive man, and what he was doing was very strange and very eccentric," says Stapledon as we prepare to make our descent.

They have finally landed, however, on what they believe is a solid theory. "If you can find more information that's better than what we've got, then good luck to you."

'Long-lost cellar system'

We walk over to the Paddington site, the largest of the dozen or so known sections of the tunnels to be uncovered.

"We were searching for a way into the long-lost cellar system below these long-lost buildings in 1999" and with the help of construction workers building student housing on the site next door, "we managed to find a way to get in," says Stapledon.

We climb down by ladder into the first of a series of large chambers hand-carved from the local sandstone and meticulously finished with vaulted brickwork ceilings. It's dry, well-lit, and altogether comfortable, like an elegant wine cellar.

Portraits of Williamson and his wife Elizabeth — he a round-faced man with a quizzical stare, she with a soft expression and cloud of silver hair — greet us at the end of the room.

Williamson was born in Yorkshire but moved to Liverpool at the age of 11 to train as a tobacco merchant under Elizabeth's father, Richard Tate. Williamson took over the business in 1803, having amassed considerable wealth through this and other ventures. He and Elizabeth remained childless, however, with no heirs to whom they could pass their fortune.

It quickly becomes apparent that calling this labyrinth "the Williamson tunnels" does this underground world a disservice. From this chamber alone, we have three onward routes: a pitch-black narrow cul-de-sac reminiscent of a prehistoric burial chamber, steps leading down into more chambers below, and the one we opt for first, into a room filled with displays of 19th- and 20th-century crockery, bottles and creepy little doll heads.

These are all finds from the excavation work that began when the tunnels, long filled in with rubble and debris, were rediscovered in the 1990s. The items on display — and even the tunnels themselves — give us a window into the past.

In the early 19th century, Liverpool was a booming maritime city, although its wealth stayed with the merchant class and casual work was often a hard-scrabble life around the docks. It was a time of mass unemployment, as the mechanization of the Industrial Revolution put artisans out of work and the end of the Napoleonic Wars in 1815 brought in a wave of decommissioned soldiers.

The grandeur of the city's history can still be seen in the epic sweep of Liverpool's wide public spaces and spectacular diversity of architecture from the Georgian era onwards. When Joseph Williamson bought land in Edge Hill in 1806, it was with an eye to creating a middle class suburb in what was then an undeveloped area some two or three miles from the city.

He built "houses all along this street. But he was also quarrying out the sandstone because it's solid, good hard red sandstone under our feet everywhere here," explains Stapledon. "And he spent 34 years up here having his men digging out the sandstone and building houses over the top in many cases."

This decades-long building project was, however, conducted in complete secrecy. Williamson never spoke of his plans publicly, was never known to invite anybody to see what was happening, and left no written records or maps that might shed light on the tunnels' purpose or even their extent.

"The men who worked for him were presumably" instructed to maintain silence and "what was happening underground was only discovered a few years after he died," says Stapledon.

"A few years after that, it all got filled up with rubbish and demolition material from the buildings up above, and it was all forgotten about and slipped into folk memory."

Interest was revived just over a century ago after a series of articles by the historian Charles Hand was the catalyst for official surveys being carried out in 1926. The Joseph Williamson Society was founded in 1989, giving rise eventually to the two splinter groups preserving the site today.

The Banqueting Hall

Perhaps aptly for a tangled warren both mysterious and frustratingly incomplete, the tunnels are, confusingly, managed by two separate organizations with very similar names.

There are the Friends of Williamson Tunnels (FoWT) and, just down the road in stableyards built after the merchant's death, the Williamson Tunnels Heritage Centre.

FoWT has by far the more impressive range of sites and the lengthier tour, but its rival boasts a small visitor center. With both operations offering tours at different times, a completist can maximize their experience by visiting both.

Of all the tunnel sites that have been identified and uncovered, the most spectacular by far is the FoWT's 27-foot-high "Banqueting Hall."

We enter this cathedral-like space from the top level, descending the scaffolding steps to explore the 65-foot-long expanse below, our voices echoing up to the gothic arch above.

The floor we stand on isn't even a floor but a layer of dense rubble that the Friends of Williamson Tunnels estimates might be 20 feet deep at its thickest. The excavations in this room alone have been decades of painstaking labor by the volunteers.

The banqueting hall earns its name from a likely apocryphal tale that Williamson had once invited well-to-do guests to his home but served them a humble meal of bacon and beans. After many left in disgust, it was claimed he then treated the loyal visitors to a grand feast in a banqueting hall.

'A strange man in many ways'

"Our source for a lot of the larger-than-life stories about Williamson is a fella called John Stonehouse," who was what we'd today call "a pop historian," says tour guide and volunteer Billy Moran, while leading a tour of the double and triple-decker tunnels at the Williamson Tunnels Heritage Centre.

These stories range from claims that he attended church with a horse whose hooves he set upon his own shoulders (a mind-boggling feat of interspecies acrobatics), to a colorful encounter with railway workers engaged in building what is now the West Coast Main Line from Liverpool to London, which cuts through the Edge Hill site.

This was the 1830s and still the early days of locomotive transport. "The railway men were getting a bit worried, a bit superstitious, apparently thinking they were digging too deep and getting too close to Hell. And they go through into this weird tunnel they weren't expecting," says Moran.

A team of Williamson's men had been lying in wait, "and they jump out, covered in mud, and screaming. Then, last of all, Joseph Williamson climbed out, started shaking people by the hand, and saying, 'Welcome to Edge Hill. If you want to learn more about tunnels, we can give you a lesson.'" Moran goes on. "He was a strange man in many ways."

'Digging holes for the local weirdo'

So what was he up to? The conclusion both Williamson organizations have come to is the same. The most likely explanation for the endless tunnel-building is philanthropy: providing honest work and skill-building for men at a time when living was tough.

"He got to a stage in his life where everything was about just keeping men busy, men who needed work, who would otherwise be standing around on street corners, getting into trouble, and ending up in prison, not being able to feed their families," says Stapledon.

As his buildings went up in Edge Hill, his men were digging deep underneath the firm sandstone rock offering a stability where other tunnels might collapse. Sometimes, men were simply digging holes and filling them back up again.

It took years of volunteers clearing out these tunnels, Stapledon says, for them to realize how deep the labyrinth went and thus to work out a solution to this centuries-old puzzle.

"No way it could have been a commercial proposition by that time. It was too silly."

Moran notes on his tour that a church local to Edge Hill has baptismal records going back to the time of Joseph Williamson and a number of grateful families named their children in his honor. Says Moran, "He was doing a lot for people who just did not have a better option than digging holes for the local weirdo."

In Italy, they have a slang word, "umarell," for men of retirement age who gather at construction sites to watch builders at work. Universally, for centuries, tunnels and digging have held a deep fascination for many.

The atmosphere at the Friends of Williamson's Tunnels construction trailer is convivial, with volunteers meeting weekly to share their passion for the project and for local history. Two of the team, Mike Rogers and Clare O'Keefe, have brought along cakes to share with the group — Rogers is known for his "Great British Bake Off" level of cupcake innovation.

They run their tours for visitors every Wednesday and Sunday while the Williamson Tunnels Heritage Centre is open Fridays, Saturday and Sundays.

As Stapledon remarks to a visitor during the tour, in the early 19th century Williamson's tunnels were a way of keeping people occupied. But he adds, "even now, it's true, he is keeping men busy."