Railway lines beneath the Clyde, the rooftops of the City Chambers and a walk beneath George Square are all up for discovery by a group of urban explorers delving into Glasgow's past

• The group walk across a graffiti-covered bridge on Whitevale Street, the Barras and Ronnie Scott at the necropolis

This is hiking Hidden Glasgow style, an al fresco history lesson organised by members of the thriving internet forum dedicated to the forgotten corners and secret past of the city. Instead of hoping to spot a kestrel, or find a bonny spot to enjoy our sandwiches, we are looking at the built environment for period remnants, architectural puzzles and clues to the past. No sitting around eating sandwiches. We might, if there is time, grab a drink in a picturesque old boozer.

Today's route takes us along Duke Street, past an abandoned abattoir, a former maternity hospital (now converted into flats) and Geronimo's Play Centre. Each one is the subject of conversation, observation and debate. There is, if you know where to look, a railway line around the back of the bleak, deserted cattle stalls, with a listed railway station. Was my father born in that hospital? And could the charmless Geronimo's, housed in a mini strip mall-style shed, be so named in honour of Buffalo Bill?

It is the search for a statue of the bold Bill that takes us off the main drag, up the tenemented sidestreets of Denistoun. Turns out that Colonel William Cody and his wild west show spent the winter of 1891 in the area, complete with entourage of horses, native Americans and Annie Oakley, turning out two performances a day. There is now a monument commemorating this little-known period in the city's cultural history in the gardens of a block of new flats in Whitehill Street.

• Some wall art in Calton; in memory of William Cody's Wild West show

Ronnie has gone ahead to mark out the route. He pops up in the distance, waving his arms, and we stragglers stomp up the hill, photograph the statue and scour the accompanying plaque for factual and grammatical inaccuracies. Ronnie sketches in the geography of Denistoun at the turn of the last century and the East End Exhibition that once stood on the site while Petra and Karen observe Bill, nonchalant on his bucking bronco, from a thoughtfully positioned bench.

Josef cannot work up much enthusiasm for the long-departed showman. He knows an unusual brick-built church in a street nearby. It is not like anything he has ever seen before. Ears prick up. Why don't we all go and have a look at it? Josef leads the way, across the main drag, past charity shops and kebab dives, round another tenement corner towards a most unGlaswegian-looking structure. It is not quite Howard Carter entering the tomb of Tutankhamun, but for a group of people whose idea of fun is identifying captionless photographs of pre-war closes, this is heady stuff.

What is this large, well-preserved, architecturally incongruous church with attached manse (to say nothing of a double garage that would be desirable in Bearsden) doing deep in room-and-kitchenland? There are no signs of life, noticeboards, names of architects or other illuminating details to help us out.

As we wander around taking photographs, a neighbour flings up her window to let us know that mass is at 6pm. Sadly she can't help with questions of a technical rather than spiritually redemptive nature. Then, just as we decide to move on, the priest pops his head around the front door. We have inadvertently set off a security light and he looks mildly alarmed at finding a disparate group of architectural thrill- seekers snooping around his front garden on a Saturday afternoon. Turns out the church was one of the first buildings commissioned by Jack Coia, of the modernist architecture practise Gillespie Kidd and Coia, in 1933. Would we like to come in and have a look around? Vigorous nods and smiles all round, and promises to return soon for the full tour. That is, says Josef as the priest watches us leave the property, typical of people's reaction to Hidden Glasgow.

• Flats on Duke Street; Geronimo's play centre;

wall details from the Necropolis; an old shop sign on Duke Street

We are further down the road, heading towards a tasty abandoned swimming pool, when it dawns that we don't know the church's name.

Given Hidden Glasgow's history, it's astonishing that St Anne's (the first of 30 Catholic churches built by Coia, red brick with white stone dressing, in a modern style heavily influenced by Byzantine and Renaissance architecture, thank you Google) came as a such a surprise to the assembled group. The website was set up in the summer of 2001 by Sharon Halliday and her partner Iain Johnstone. Both were new to the city and curious about its industrial past. For Halliday, a web designer, the obvious place to find answers to her many, many questions was the internet. At the time there was no site for people to post pictures or instigate discussions about abandoned doocots, intriguing chimneys or abandoned football grounds. So she set one up. A year later she set up the forums that are now the group's lifeblood.

Hidden Glasgow soon moved from the virtual world into the real one. In the early years the group's main focus was urban exploration, identifying buildings that were empty, derelict and often awaiting demolition, then finding a way into them. They have been visiting the crumbling wreck of St Peter's Seminary in Cardross, a much later Gillespie Kidd and Coia building, and documenting its deterioration since the site started.

They managed to see round Glasgow Granary, the towering grain store that dominated the pre-Glasgow Harbour Clyde waterfront, before it was flattened. More recently they descended on the Bell Street stables, a curious multi-storey living space for carthorses which, after being used as a council refuse depot, then lying empty for 15 years, is now about to be turned into flats.

These days, however, the hardcore urban explorers have moved on to a couple of more dedicated websites, leaving Hidden Glasgow a more generalist venue for (still pretty arcane) debate and discussion, the posting of many, many photographs and the organisation of walks that do not go so near the wild side. They are not averse to popping into an abandoned building if they happen to pass one by, but it is not their raison d'etre. The group has evolved to include influential and well-connected members who know how to arrange access to the secret passageways and underground tunnels. Lately they have had a tour of the railway lines beneath the Clyde, a nosey around the rooftops of the City Chambers and a walk beneath George Square. Just by asking nicely.

They are observers and recorders, not adven- turers or campaigners.

Different members bring their own obsessions and passions to the group. There is not much Ronnie does not know about Glasgow's sculptures and graveyards. Scott, who lives in Edinburgh and regularly travels west to poke around the back corners of the rival city, is particularly interested in iron railings and street lights.

Some prefer their structures intact, where others love the plangency and decayed glamour of a wreck. Euan, another east coaster, is underwhelmed by the pristine brickwork of St Anne's. "I just prefer more dilapidated buildings."

The group has walked along the route of the M74 extension once a year since it was first mooted. "We started when the compulsory purchase orders had gone in but they hadn't started demolishing yet, to the present, when it's more or less built," says Josef. "We do the walk on 2 January when security guards tend to be asleep or not there at all. And there's always a hole in the fence."

One year, passing an empty mattress factory, they jinked in to have a look around. Another year they diverted off into a graveyard. "Most Hidden Glasgow walks involve a graveyard at some point." The road is now almost completed and the website has a comprehensive archive of the minutiae of the domestic and industrial architecture of Oatlands and surrounding areas, now gone forever.

Other recent jaunts have followed the course of the River Cart and explored the sights of Springburn. Usually a member with a particular interest or connection to an area will suggest a walk, do a bit of research and plan a route. Some may print out old pictures of the area to do a compare-and-contrast, or just do a bit of swotting to get an idea of what might be lurking behind the Barratt estate or new-build shopping centre. Petra, an Austrian living in Glasgow, may well bring a flask of hunter's tea (it's a drizzle-beating mix of Tetley's, wine and rum).

Wandering around the back lots of Glasgow's wild east is not for the faint of heart. At least in the Necropolis, the magnificent Victorian garden cemetery beside the city's cathedral, we are not the only visitors with serious cameras and sturdy footwear. Ronnie, who has written a book on the place, can point out the Grand Master of the Texan freemasons and the first bandmaster of the Salvation Army without looking out of place. But out on the streets of the Calton, as the Parkhead crowd straggles home, we are middle class and incongruous among the windowless bars and boarded-up shops. Ronnie may be wearing a hoodie, but it bears the legend 'University of Strathclyde'.

In the narrow pavements around the edge of the Barras, it's clear we are not there to buy sectarian underpants or hooky DVDs. As the stallholders pack up, we cause an obstruction by stopping to admire a bicycle stall's ancient tandem and the tiny upper doorway cut out of a brick wall. An older couple, who may well have passed a jolly afternoon in a neighbouring tavern, attempt to pass. "Are you tourists?" the man inquires. There is general murmuring and demurring, during which no one quite admits to being the kind of person who chooses to spend their Saturday afternoon cataloguing the interior structures of a covered market. "Of course they're not tourists," spits his wife, taking his arm, huckling him homewards, apologising over her shoulder.

This is, according to the veterans of Hidden Glasgow outings, entry-level stuff. The group's recent day out in Springburn caused great consternation among the locals. "They asked us, are youse a football team?" recalls Scott. The accents also floored them, what with Petra being Austrian and Karen, another regular, Irish. "Are youse Australian?" Although the most common one they get, when exploring a particularly unlikely scheme is, "are you lost?"

"I don't suppose," says Scott thoughtfully, "that Springburn gets many visitors."

Yet there is, Josef insists, something interesting pretty much everywhere. All you have to do is look for it, which is why the site's motto is " always look twice". He was recruited via Google: "Whenever I looked up anything in Glasgow, anything remotely obscure, Hidden Glasgow came up every time."

What he was Googling was, he says, often "sheer nostalgia. It's not just about things that are still there and people don't know about, it's things that used to be there and lots of people know about, from the very recent past. So much of Glasgow has disappeared in the last 40 years. Even the last ten years. Before Google Streetview arrived I thought I ought to go out and photograph everything because it might not be there next week. How often do you go down a street and one day there's a building missing. You ask yourself, what used to be there? You can never remember and that sort of archiving thing is what Hidden Glasgow does.

"My first memory of Glasgow is being driven through the Gorbals in my father's car. It was this complete urban wasteland with a little pub on each corner, a one-storey box. They had bought out the tenements and demolished them but they couldn't afford to buy the pubs. That happened in a lot of areas.

"I've spent a total of 15 years here, on and off. I've lived in few different parts and some are more prone to change than others. The rich bits tend not to change very much, Hyndland and the west end stay the same but the east end and the north, they've wiped parts of that entirely even since I was an adult.

"My first ever flat was in the east end, an art deco tenement over near Parkhead. It's now gone entirely. And that's not uncommon."

Unlike many of the site's stalwarts, Josef has no specific speciality. It seems to be the thousands of tiny fascinating details that make up the city, the named bricks from specific brickworks, the tram rosettes, that keep him coming back.

"Glasgow is a fascinating place. I'm not sure myself whether I love it or hate it. No matter what, it fascinates me and I'm always happy to find out more about it."

It's not, he insists, a nerd thing. "Geekishness tends to get hived off into sub-sites. We don't tend to have a geekish membership." (This after a detailed description of the shale extraction industry and the brickworks that grew up around it.)

What Hidden Glasgow has is a membership that loves a puzzle. Josef's face lights up as he describes a captionless photo of a tenement, taken by a member's grandfather, posted on the site for identification.

"It turned out to be, after 35 pages of comments, a long gone bit of Maryhill Road. The effort put into identifying that as Maryhill Road was extraordinary. It came down to markings on chimneys and chimney patterns all the way along the road. People were digging out satellite photos and photos from 1945 and comparing the chimneys and the banding patterns on the tenements. We were sitting up till two in the morning on 3 January trying to work out where the f*** this place was. It wasn't so much an interest in architecture or old Glasgow as such. It was a puzzle and people wanted to solve it."

Five miles later, back on Alexandra Parade, I think I spot my first tram rosette. I have lived in Glasgow for 25 years without being bothered by these ornate metal hooks at first floor level of tenement buildings, put there to carry the trams' electric cables. I see a bit of curly wrought ironwork above a newsagent and whisper to Josef. Is that a tram rosette? Six people turn on me. No! It is of a much later vintage and would once have held a shop sign. That – they point in unison to an undistinguished rusty item further along the block – is a tram rosette.

Later on, at home, I look up tram rosette on the Hidden Glasgow website. Lots of fascinating pictures and debate about why they seem to cause cracks on the stonework. Somehow half an hour has gone by.

Of course it's not a geekish site. Not geekish all.

www.hiddenglasgow.com

• This article was first published in the Scotland on Sunday on October 12, 2010