In a sprawling city of almost nine million inhabitants, there are few sites that remain unseen. Few corners that remain unfamiliar, and few places that are not worn into their own groove by a daily ritual of crowds, workers, tourists, coffee cups, bus stops, fast food wrappers, years of excess, years of recession, free newspapers, couriers, the beep of horns, the beating heat of central London in summer, the last train home, the first train back.

Yet corners remain. Gigantic, cavernous corners forming the negative space to one of London’s most famous landmarks: Tower Bridge. Unheard, hidden beneath by the water, deep inside one of the most recognisable bridges in the world, lies the bascule chambers. When the bridge’s arms are lifted, the bascule (French for swing) chambers house Tower Bridge’s huge counterweights. When the bridge is at rest, the gigantic brick-lined space stands empty: a dramatic vault of an unknown, subterranean space, and home to unique acoustics.

For the second year running, as part of the Totally Thames festival, internationally renowned composer Iain Chambers returns for a run of specially-commissioned performances within the chamber. This year the award-winning Juice Vocal Ensemble will take to the stage, joined by Ben See Group, combining classic and contemporary music set against an unforgettable backdrop of a great feat of Victorian engineering.

What’s it like to compose music for one of the last hidden vaults of a metropolis? In the run-up to the performances, Run Riot spoke to Iain about working with such an extraordinary location, as well as examining how Iain collects sounds, texts and moments to shape his compositions.

RR: How do you describe the music you create?

IC: In terms of the music I've written for the bascule chamber, it's site-specific music written with that particular space in mind. More generally my music often uses field recordings and location sounds as compositional material. I'm particularly inspired by the ambient sound around us in our everyday lives.

RR: Do you collect sounds? In your mind, are they a similar entity as ideas to writers? Do you have to record them and keep them so you can use them or go back to find them?

IC: I do collect sounds. I'm particularly interested in anything with recognisable pitches that I might be able to compose with and manipulate, but I'm also concerned with not over-working the sounds I record: I want the source material to remain recognisable. Particularly resonant sounds are inspirational, and often trigger whole passages of music in my head. I have endless notebooks in which I note down good potential source material that I've spotted, then it's just a case of getting back to it with a recorder before the sound changes, or the building is demolished...

RR: Was there ever a ‘sound that got away’ for you?

IC: I used to work in Broadcasting House in Portland Place, and the engineers who oiled squeaky door hinges were the bain of our lives. Often you'd return to a particularly fruity door only to find it had been 'fixed'. It felt like a bereavement. Then there's the platform announcer at Moorgate Underground. I missed recording her spectacularly cheerful sing-song delivery before she was moved to another station.

RR: Where is the best place in London for sound? Where is the worst?

IC: The worst place for me is where ever the sound of planes or cars overwhelms your recording. Best places include Arnold Circus in Shoreditch, any cemetery or wild place, and the strange village-y streets like Mile End Place or Bellevue Place, where the comparative quiet is a big surprise.

RR: Is there somewhere in the world you’re yet to visit, that you’d like to go to listen to?

IC: Trevor Cox's book Sonic Wonderland is like a Yellow Pages of great sonic phenomena that I'd like to spend my life working through, recording and composing with.

RR: It’s amazing that so few people seem to know of the Bascule Chambers, and that there is so little information online about them (and that they haven’t been used for a sequence in a thriller film yet!). How did these performances originally come into being?

IC: It's startling that so many people travel over Tower Bridge every day, oblivious to the amazing space beneath them, and certainly unaware of the incredible sound the bridge makes in the bascule chambers whenever it lifts up. It's one of the great sonic wonders of the world! I first came across it when I heard the recording by London Sound Survey, and I immediately thought it was a piece of music waiting to be realised.

RR: When you’re writing for the bascule chambers, do you set out to capture a certain atmosphere, or is it more reflexive than that? How do you prepare for a performance in such a unique setting? For the 2015 compositions you worked with selected texts to inform your work, will you be doing something similar for 2016?

IC: I'm certainly trying to write material that I think will resonate in the space and not get lost. It's a great moment when you hear your new piece performed for the first time in the space, with all the amazing off-stage sounds part of the mix, from passing boats to road traffic and the calls of sea birds. Plus it's a thrill to witness members of the public stepping into the bascule chamber for the first time and being overwhelmed by it. This year I'm again setting poems by Emily Dickinson for Juice Vocal Ensemble and Ben See Group to sing. There's something about Emily Dickinson's work which particularly chimes with the bascule chamber. It's meditative, sometimes quite way-out, and often wistful and beautiful.

RR: Who are your favourite composers?

IC: The composer I've found most inspiring recently is completely unknown in the UK. He's a German sound artist called Andreas Bick whose radiophonic pieces are so beautiful and inventive. I also love the music of the British composers Howard Skempton and Trevor Wishart, alongside the French composers Bernard Parmegiani and Francois Bayle.

RR: Are there any artists you’d love to work with?

IC: Julia Holter – if she ever needed some location-based musique concrete for her albums, I would be persuadable.

RR: Is there a film or television series you’d like to re-design the score for?

IC: Top Gear's troubles might not be resolved by a new score, but it would be a good start. Meanwhile on radio, to replace the Archers' theme tune with.... anything else, would be an act of mercy for many sufferers.

Iain Chambers’ Bascule Chamber Concerts are now entirely sold out. But the good news is the Totally Thames Festival has plenty of other attractions taking place across every day in September. Estuary 2016 will be producing 16 days of art, literature, music and film, alongside London’s Burning, a festival of arts and ideas commemorating the 350th anniversary of the Great Fire of London. Highlights include the Ladies Bridge Film Screening and Panel Discussion on 22nd September at BFI Southbank, the Thames River Swim on 7th September and Floating Dreams, a major installation by Ik-Joong Kang, one of South Korea’s most celebrated multimedia artists, situated in the centre of the River Thames by Millennium Bridge.

Full details here, and more information on Totally Thames here. Iain Chambers’ website is here and you can tweet him at @mriainchambers, and you can follow the festival on @TotallyThames and using the #TotallyThames hashtag.