Under the streets of Liverpool, England’s Edge Hill district tunnels stretch for miles. The residents know they were built between 1810 and 1840 by eccentric local business man, Joseph Williamson, but no one knows their true purpose, reports Chris Baraniuk for BBC. And only in the last 15 years have people begun to dig out the more than a century’s worth of junk and rubble and explore the network.

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“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,” says Les Coe, a member of the Friends of Williamson’s Tunnels group, which formed in 1996 as an off-shoot organization of the Joseph Williamson Society. In the group’s excavation work, they’ve found cellar systems, areas with multiple tunnel levels and more than 120 trash skips full of debris from decades of people using the tunnels as a 'out of sight, out of mind’ waste disposal.

But not all of the finds are trash. Baranuik writes:

As they excavate, the volunteers methodically document any artifacts 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.

Theories on the reason that Williamson had the tunnels built abound. Perhaps he was a smuggler that used the tunnels to move goods. Or one theory holds that "his wife came under the influence of a lunatic preacher who told her the apocalypse was coming and she persuaded Joseph to prepare chambers for underground living," Gordon Hunter, the chairman for the FoWT tells Emma Kennedy in a story for The Guardian. Or maybe the tunnels’ construction was merely an ongoing but ultimately pointless source of employment for locals.

Dave Bridson, manager at the Williamson Tunnels Heritage Centre in Liverpool, tells Baraniuk that he thinks that the tunnels are reclamation work that allowed people to build houses above the pits remaining from an old sandstone quarry. Instead of filling in the pits, Williamson had his men build arches.

Even if the reason for the tunnels is never discovered, they’ll continue to fascinate people. As of now, their mystery stands with the likes of the tunnels beneath the old Roman spa of Baiæ (Baia). Even if solved, the allure of old tunnels is sure to draw interest (just as it does to the catacombs beneath Paris and the series of passageways beneath Washington D.C.

Plus, giving the tunnels some airing out can’t hurt: They were originally filled in and sealed from the outside world decades ago when locals complained of the smells wafting from below, as Baraniuk writes. It was simply the inevitable consequence of creating underground landfills under city streets. Fortunately, time seems to have tempered that problem.