It is the river that gave the city its name, yet few in Sheffield appear even to have heard of it.
The Sheaf, around whose confluence with the Don the place was built, was largely buried by the Victorians – an unwanted waterway enclosed in culverts that converge in an enormous underground “cathedral”
It is seen usually only by an army of adventurers, urban counterparts to the potholers of the Yorkshire Dales, who post videos of themselves in their private, subterranean city.
Now, however, the vast brick tunnels are being briefly opened to the public, along with proposals to reinvent the river for the post-industrial age.
A series of “urban caving” tours is being laid on as part of the Sheffield Adventure Film Festival, a weekend-long event later this month which is otherwise conducted in the comfort of a cinema.
“It’s part of the special characteristic of Sheffield that you don’t necessarily have to go out to the wild countryside to take part in adventure sports – you can do it in the city centre,” said Simon Ogden, a town planner who chairs the Sheffield Waterways Strategy Group.
But in a twist of irony, the bulk of the festival will take place in an auditorium that is itself hiding the Sheaf from view.
The Showroom Cinema, named after a car dealer that occupied the premises in the 1920s, was built directly over Porter Brook, another of Sheffield’s “lost” waterways.
“There is some irony in that, absolutely,” Mr Ogden said.
“The building has been there since the bad old days when they were still culverting the rivers.”
The Don and the Sheaf – too fast to be navigable but powerful enough to drive Victorian engines – converge beneath Castlegate in the city centre, with the Sheaf remaining underground until it emerges at the Blonk Street bridge.
“Most people aren’t aware of it because it’s invisible in the city centre,” Mr Ogden said. “There’s only one very short section next to the bus station, with fairly high walls around it. So people in Sheffield just don’t know about the river it’s named after.”
The huge space at the centre of the tunnel network, known locally as the Megatron, rises up “like a cathedral”, Mr Ogden said.
An initiative to redevelop the Castlegate area would see the section of waterway that connects it to the Don opened up in a landscaped area that would be called Sheaf Field.
“It’s underneath the station, which obviously isn’t likely to be moved in the foreseeable future, but there are plans to alter it for HS2 and that presents opportunities to improve the river and maybe get lights down to the river culverts,” he said.
“We’ve already done a little park down at Matilda Street, where river has been re-naturalised and you can now see trout in there. There are opportunities to do that on other parts of the Sheaf.”
Officials hope the film festival and the tours of the tunnels will encourage people to engage in what Mr Ogden termed “environmental activism”.
He said: “It’s about encouraging people who enjoy the environment in one of those sports to actually put something back – with a particular focus on helping to clean up some of the rivers.”