Video report by ITV Wales reporter Mike Griffiths
Image supplied by Graeme Bickerdike
It has been hailed as a feat of Victorian engineering - but the Rhondda and Swansea Bay railway tunnel hasn't seen a train pass through for more than half a century.
Now, campaigners are hoping to connect communities in the Rhondda and Afan valleys by reopening the tunnel as a route for walkers and cyclists.
It is the longest disused tunnel in Wales at 3,443 yards, or nearly two miles.
If it secures enough money to be developed, it will be the longest cycling tunnel in Europe and the second longest in the world.
The tunnel was temporarily closed on safety grounds in 1968 but never reopened, after the cost of repairs were deemed prohibitive.
Following years of disuse, both of the Rhondda Tunnel’s portals were finally covered over and landscaped, with access limited to survey teams and the campaigners trying to save it.
The tunnel is sealed at both ends, so visitors have to descend 60ft down an old air shaft with help from experts.
But once inside, campaigners say it is in a similar condition to when it first opened back in 1890.
The Rhondda Tunnel Society was established in 2014 and has more than 850 members, with Welsh actor Michael Sheen among those backing the project.
Tony Moon, from the Rhondda Tunnel Society, remembers his first visit.
"It was a wonderful experience," he said.
"Walking through the tunnel and seeing this magnificent buried tunnel that no one had been seeing for a long, long time - and in pristine condition. There was a little bit of soot on the walls, but that's about all.
"We just want so many people to come and see this wonderful piece of Victorian engineering. This relic has survived, and it will tell future generations so much about life here. Joining the two valleys up - joining two communities together, which were cut off from one another when the tunnel was shut."
Tony estimates he has been down into the tunnel around 30 times in total.
"I know it pretty well - but I love it every time," he added.
Councillor Gareth Wyn Hughes, from Rhondda Cynon Taf Council, said campaigners had been "doggedly determined" in their ambitions to reopen the tunnel.
"I think it would be fantastic to see the tunnel open, connecting the upper Rhondda, the upper Gwynfi valleys.
"But also if we can build on it - building on the success of things like Zip World in Rhigos."
Reopening the tunnel and turning it into a visitor attraction could cost more than £10 million - but campaigners believe it has a bright future.
"When I went in for the first time, which was 2015, I can remember shouting: 'Hello, my old friend' - it seemed like an old friend," said Steve Mackey, of the Rhondda Tunnel Society.
"The challenges that still remain is the ownership of the tunnel, and, of course, to excavate the portals. But instead of digging inwards, we'll be digging from the inside outwards, so that's going to save us a lot of money as well."
The tunnel is currently under the ownership of the Department for Transport. Talks between the Welsh Government and the UK Government about handing the tunnel over are expected to continue.
A mission statement on the Rhondda Tunnel Society website reads: "It is time for it to be recognised for what it is, what it stands for, and how this tunnel can help bring much needed tourists to these parts of the Valleys.
"Blaencwm and the Afan Valley are of great beauty these days; the walks are spectacular. Let’s keep it that way and even get it better."