Writer Karl Whitney takes a fascinating look at the underground history of Dublin city
Beside the primary school I attended, St Mary's National School on the Greenhills Road in Tallaght, is an area of ground now largely occupied by a gym, a five-a-side football centre and a running track.
But in my day it was a large flat field adjoined by houses to the south and an industrial estate to the north. These open spaces formed part of the landscapes of my childhood and the places in which so many of us grew up.
Through the middle of the field flowed a small sewer-like stream that often smelled of chemicals from the factories upstream: the Poddle.
On sports days or nature walks we'd jump the stream - it wasn't that wide. I can't recall anyone falling in, but we knew it was polluted. The water was dirty - not to be touched. It was better to ignore the river, it seemed. But I was intrigued by it, yet it was many years before I properly followed up this interest.
The Poddle didn't seem important and visible, at the centre of things, like the Liffey. Instead, it was on the edge of things, emerging here through a fairly unexceptional field before disappearing farther east between suburban houses and beside churchyards before disappearing beneath the city, a hidden river.
Years later I was surprised to meet the river in James Joyce's Ulysses: Joyce wrote that "from its sluice in Wood Quay wall under Tom Devan's office Poddle river hung out in fealty a tongue of liquid sewage". (Actually, it emerges a little downstream, through an arch in the wall of Wellington Quay that's visible from the Millennium footbridge.)
Today the Poddle is much cleaner than it was in Joyce's day. But what I couldn't figure out was where the sewage goes now? I set out to follow it through the pipes and treatment plants to the sea when I was researching my book Hidden City.
Sewage is kept out of sight - we try not to think about it. We happily go through days, weeks, months and years without thinking about the narrow channels of human waste flowing slowly beneath every street in Dublin.
But I took the unusual step of spending a couple of months researching its history, talking to drainage workers about their job and even taking a tour of Dublin's main sewage plant, a largely anonymous collection of buildings near the ESB's famous Poolbeg chimneys at Pigeon House.
I began by looking at a map that showed all the places in Ireland where sewage, in a variety of states, from untreated to highly-treated, emerges into bodies of water (lakes, rivers or the sea).
I talked to Alan Vickers of Dublin City Council, who showed me how the council tracks sewage through the pipes using underground sensors. We took a look down a few sewers: small suburban pipes and huge trunk sewers that carry large volumes of sewage towards the treatment plant.
I saw huge tanks holding liquid that had been flushed a matter of hours beforehand at the treatment plant. I was shown a skip which held grit that had been skimmed from the sewage so that it wouldn't damage the treatment plant's machinery.
Among the grit were pieces of sweetcorn that had survived the trip through the pipes - and the journey through Dubliners' digestive systems.
I'd come a long way from the smelly river that ran next to my school, but I still wanted to find out more about the Poddle. One day I met workers from Dublin City Council and we climbed down a manhole into the river near Leo Burdock's chip shop in Werburgh Street. After I reached the bottom of the ladder, I stepped into the Poddle, which flowed through the tunnel in a depth of a couple of inches.
The water was clear and didn't smell of anything - not even of the chemicals that had once polluted it as it ran past my primary school.
We walked through the dark tunnels upstream, in the direction of St Patrick's Cathedral. A ladder had been lowered near the cathedral to allow us to climb out.
But we ignored the ladder for the moment - instead, we continued down a narrow side tunnel built from stone. At the end of that tunnel, we reached a culvert through which raw sewage flowed.
Even though the Poddle, once seen as an open sewer, had cleaned up its act and sewage had been diverted elsewhere, the river and the sewage system were still connected here by the network of tunnels, a reminder of the underground history of Dublin and of the hidden city beneath our feet.
Hidden City: Adventures and Explorations in Dublin, by Karl Whitney, is published by Penguin Ireland, price €19.99.