It’s a busy three-lane bridge that hundreds of people drive and walk over every single day, into and out of Wales’s oldest town.
Many have been on it, many have been under it, but how many people can say they have actually been inside it?
Carmarthen Bridge spans over the River Towy at the foot of a hill that leads up to a former county jailhouse which continues to loom above everything else.
But what on first glance appears to be just an ordinary bridge is anything but that.
Behind heavily-locked doors lies a secret room to which general access is strictly prohibited that looks like a cross between a secret MI6 bunker and a Victorian snooker hall.
There's been a bridge over this spot of the river since the 13th century. Back then it was a seven-arch structure that, after hundreds of years worth of damage caused by the elements, was on its last legs by the 1930s.
In 1933, a new three-arch bridge was designed by architect Clough Williams-Ellis - the man behind Portmeirion - which opened to the public four years later.
It made headlines across the country in October last year, when the rain-battered Towy tested its construction and almost breached its three arches.
The bridge itself remained intact, but some businesses in the adjoining Pensarn area were not so lucky, left to pick up the pieces as thousands of pounds worth of stock drowned along with their profits.
But Carmarthen Bridge was built to last, as I found when I was allowed a rare visit.
Edward Rees, structures engineer at Carmarthenshire Council, carries out regular inspections - which take a whole week to do.
"Not many people have been in here or know about it," he tells me, as he shows me around.
“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.”
Mr Rees admits to having the same feelings of wonder I'm encountering when he first set foot inside near a decade ago.
"It was about eight years ago that I first came down here," he says. "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.”
There are two separate access doors which lead to a total of three different sections of this sprawling, hidden part of Carmarthen town centre.
The three sections are known as Stores Viaduct, Station Approach and Carmarthen Bridge itself.
The first of those sits underneath the A484, which runs alongside Towy Garage. I had visions of being met with every horror one would expect from what is essentially an unoccupied and unused tunnel - spiders, rats, dust, litter. But, to my amazement, the inside is spacious, unlittered and clean. There is rubble on the floor but otherwise you could be forgiven for thinking that this vast space was used in recent years as a trendy bar or open-plan office.
It hasn’t been, sadly. It was formerly used by Carmarthenshire Council as a storage area, but is currently empty.
The best and most impressive part of this underground lair, however, is at the end of Station Approach, when a dog-leg left allows you to actually walk under Carmarthen Bridge and above the River Towy. You can even walk over the three arches and look out through tiny port holes at the water under your feet.
As you stand inside one section, you can hear the cars, the vans and the lorries pounding past above your head.
One thing it remains a home to is the memory of a devastating flood in Carmarthen in 1987, when two days of persistent rain battered the town in a way not seen for more than 100 years.
Inside the concrete walls there sits a permanent reminder of the traumatic impact of that two-day period, with a line drawn into the wall - five feet above the floor - showing just how much of this viaduct was sat beneath water 32 years ago.
I've asked the council whether this hidden piece of majesty could be put to use - as a museum, some rustic offices, a bar even?
Health and safety, among other obstacles, means that this can never happen.
That will forever be a shame, because despite the busy world being just feet above you, the wonder of Carmarthen Bridge and its interior means that, at the same time, it’s a million miles away.
