The hidden rooms inside a town centre bridge that thousands drive over every day

If you've ever been to Carmarthen, chances are that you've driven or walked over Carmarthen Bridge. At almost 100 years old, it's a three-arch structure which supports a bustling three-lane main road. But despite its ordinary appearance, this bridge, which spans the River Towy and leads up to an imposing former county jailhouse, is far from typical.

Behind heavily-locked doors you'll find a secret space reminiscent of a cross between a secret MI6 bunker and a Victorian snooker hall, strictly off-limits to the general public. The bridge's history dates back to the 13th century when a seven-arch structure stood in its place. However, after enduring centuries of elemental damage, it was on the brink of collapse by the 1930s. You can keep up to date with the latest Carmarthenshire news by signing up to the local newsletter here.

Then, in 1933, architect Clough Williams-Ellis, renowned for his work on Portmeirion, designed the new bridge that opened to the public in 1937. The bridge made national headlines in 2018 when the rain-swollen Towy almost breached its three arches. While the bridge remained intact, businesses in the adjoining Pensarn area were not as fortunate, suffering significant losses. You can read more about here.

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Despite these challenges, Carmarthen Bridge, also known as Towy Bridge, has proven its resilience, as I discovered during a rare visit in 2019. At the time, I met Edward Rees, a structures engineer at Carmarthenshire Council, whose role was to conduct regular inspections of this hidden world beneath Carmarthen town centre. "Not many people have been in here or know about it," he said. "The scale of it is hard to get your head around when you're stood outside. It really is an amazing piece of construction, and it's certainly unique that you can walk like we're walking now inside a bridge of this size.

"We have to carry out a general inspection every two years - which takes about a week in itself because of the scale of what we're dealing with. I couldn't believe it when I first saw this. I was shocked by the size of it. From an engineering point of view, it's very impressive, especially when you consider that it was all designed and built in the 1930s when there would have been a lack of modern construction equipment. Apart from some strengthening works that were carried out in the 1970s, it hasn't changed for more than 80 years."

This sprawling, hidden part of Carmarthen town centre is accessed via two separate doors, leading to three distinct sections known as Stores Viaduct, Station Approach and Carmarthen Bridge itself. The first of these is nestled beneath the A484, adjacent to Towy Garage. I had anticipated encountering all the horrors one might expect from an unoccupied and unused tunnel - spiders, rats, dust, litter.

Yet, to my surprise, the interior is spacious, clean and free of litter. There's rubble on the floor, but otherwise, you could be mistaken for thinking this vast space was recently used as a trendy bar or open-plan office. Regrettably, it hasn't been. It was previously utilised by Carmarthenshire Council as a storage area, but was vacant when we explored it.

However, the most impressive part of this subterranean lair is at the end of Station Approach, where a sharp left turn allows you to walk under Carmarthen Bridge and above the River Towy. You can even walk over the three arches and peer out through tiny portholes at the water beneath your feet. As you stand inside one section, you can hear the cars, vans and lorries thundering above your head.

One thing that remains a constant in this place is the memory of a devastating flood in Carmarthen in 1987, when two days of relentless rain ravaged the town in a manner not seen for over a century. Within the concrete walls, there's a permanent reminder of the traumatic impact of that two-day period, with a line etched into the wall - five feet above the ground - indicating just how much of this viaduct was submerged underwater 32 years ago.

I've queried the council about the potential of this hidden gem - could it be transformed into a museum, rustic offices, or even a bar? Unfortunately, due to health and safety concerns among other issues, this will never come to fruition. It's a real shame, as despite the hustle and bustle of a town centre being just inches above you, the enchanting interior of Carmarthen Bridge feels like you're a million miles away.