The Brunels made engineering history with the Thames Tunnel at Rotherhithe — but they also unwittingly created a great performance space whose potential has now been unlocked, says

shaft and the tunnel’s construction.

“It has a wonderful acoustic for opera, theatre and music: guitar, violin, klezmer, accordion,” says museum director Robert Hulse. “Although I’d steer clear of saxophones, because they tend to blow your ears off. You can get married here too.”

Laurence Pinn, the young architect who designed the cantilevered staircase, which is the main addition to the Grade II*-listed structure, says the chamber echoes the sound qualities of Irish singing halls, which are also circular. This is remarkable — the shaft was originally just a “giant pastry cutter” forced into the earth — but also oddly in keeping with the history of this pioneering infrastructure project.

Large ramps to facilitate commercial horse-drawn traffic beneath the river were never built because of cost, so the tunnel was only used by pedestrians. The entrance shaft became a grand hall and the tunnel a place of entertainment, dancing, promenading and, later, prostitution and petty crime. In 1865 the tunnel was bought by the East London Railway and the shaft became a flue for smoke from steam trains, which still blackens the brick.

The entrance at the north end was incorporated into the new Wapping Station. Later, the tunnel itself became part of the East London line. The concrete floor in the Rotherhithe shaft was put in, pro bono, by Transport for London contractors at the behest of the museum in 2008, when the tunnel was modified to carry London Overground trains.

“The space is a historic exhibit in itself,” says Jerry Tate, a partner in Tate Harmer. “It’s very raw and you can see the patina of history written on the walls. We haven’t cleaned it or tidied it at all.”

As well as the industrial grime, you can see the ghost outline of the old staircase, removed as a fire hazard when trains ran below. The changes Tate Harmer have made to the structure are sensitive: a ramp curves around the building and down to a door cut discreetly into the raw brick of the shaft, below street level. This gives access to the cavernous interior, and the fittingly muscular industrial staircase, which also provides a mezzanine platform for wheelchair