Liverpool is perhaps most associated with its historic waterfront (as the port of registry for the RMS Titanic, for instance), and for The Beatles, of course. But the city also sits atop one of the great architectural mysteries in history: a labyrinth of ancient tunnels and secret passages, all built beneath the city's streets hundreds of years ago for reasons that remain mysterious to historians today, reports the BBC.

The labyrinth, called the Williamson Tunnels after the man who designed it, hints at a suspicious, forgotten history. Its passages weave throughout the city and branch off in so many odd directions that researchers still don't know their full extent. Theories abound about the purpose of the tunnels: Were they secret smuggling routes? A hidden lair for a secret society? A bunker for members of an obscure religious cult to ride out the apocalypse?

To this day, historians can only guess, and the original purpose of the tunnels has long passed from knowledge into myth.

Here's what we do know. They were built in the early 19th century by successful tobacco merchant, Joseph Williamson. After the Napoleonic Wars, unemployment was rife across Britain, and Williamson gave back to his community by employing men in building projects. Mostly this consisted in the building of houses — and then came the tunnels.

Williamson went to the grave being extraordinarily secretive about their purpose. There are almost no records of what the tunnels were used for in Williamson's time, but following his death they mostly fell into disrepair. The caverns were first used as underground landfills, stuffed with everything from household junk to human waste. Eventually they were buried after residents complained of the smell emanating from beneath the city. From there, they became little more than legend, mostly forgotten by time.

It wasn't until 2001 that a small band of investigators decided to do some digging, hoping to sift truth from legend. They started by excavating a suspected tunnel in the Paddington area of Edge Hill, a borough in Liverpool where Williamson resided and where the tunnels were storied to branch out from. What they found was extraordinary: a large chamber, seemingly an entryway into the passages. As they cleared the debris, they found more passages leading to more passages, stone stairwells that descended into the rubble, hinting at deeper caverns, many of which still remain buried even after almost 15 years of digging.

It's impossible to know the full extent of the tunnels, as many of them have yet to be excavated or even discovered. The wealth of artifacts and archeological knowledge is substantial, however. Some of the items uncovered include inkwells 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. The tunnels are a time capsule for the city of Liverpool, a record of the lives and livelihoods of its residents, generation after generation.

The tunnels that have been restored are quite architecturally extraordinary. Supported by arches, the chambers and passageways represent a forward-thinking design that came years before the great railway tunnels and bridges of England were ever conceived. Their structural integrity is so strong that they remain standing over 200 years since being built despite receiving no maintenance whatsoever.

Today, water droplets seep through the porous stone, and as the tunnels receive their first light in hundreds of years, moss and ferns have begun to grow in various corners and along the walls, adding to the eerie mystique of the timeworn labyrinth.

Though many of the labyrinth's secrets remain elusive, the mystery remains a large part of its allure. The enigma is motivation that continues to draw in volunteer excavators, driven by curiosity and the haze of history. Perhaps it's fitting that 200 years after they were built, Williamson's tunnels are still keeping the residents of Liverpool hard at work. Perhaps that is the true purpose of the tunnels, after all: to excite our imaginations, to keep us delving into its depths, and to keep us curiously seeking, and searching, and wondering, though we know not what for.

If you're interested in learning more, or perhaps visiting yourself, check out the website for the Friends of Williamson's Tunnels here.