I was the edge of a building site. Nothing special. There were portcabins and stacks of scaffolding poles pushed up against my edges. People walked through me.

Now, I’m a space, with a name: The Skirt Of The Black Mouth, though there aren’t many who know it. There is writing on the internet about me.

Mornings and evenings most people’s feet follow the same line they always followed, they walk past me and even when they glance over, at my trees, the long smooth bench, the dark wooden slats of my walls, they don’t stop. It is lunchtime that they come, clutching their tiny bags of food. They sit at my edges, in my shadows, and I shelter them.

The Skirt of the Black Mouth is a narrow, uneven triangle of space, reclaimed from the building site of the Tate Modern Project and the adjacent energy plant. A white concrete bench runs along two edges, backed by slats of burnt wood which allow you to see through into the building site. A line of plane trees offers shifting, flickering shade. White chalk boulders dot the space, which is also crossed by two wood-cobbled paths. It is a cut-out space, positioned on the edge of Sumner Street’s curve, almost invisible until you’re upon it. It is a temporary space, which will be absorbed into the landscaping once the building work is done.

There is a lightness; a movement as the building stretches up to make spaces where there used to be sky. It turns from the steel edges and glass planes of its neighbours, veiled by brick, its concrete simply concrete. It is a building that sleeps at night. Inside, it unwraps, opens up, takes you by surprise.

The Skirt of the Black Mouth is a fiction, a nod to the fragile territory of the same name in Brian Aldiss’s sci-fi novel Hothouse. Aldiss presents an earth – now dominated by aggressive vegetable life – that has stopped turning, and a sun rapidly heading for supernova. It is a place of danger, death and occasional beauty, where the Black Mouth sings its siren song, drawing all living things into its maw.

Climb to the top of the crane – high above the dust and the drilling, the fluorescent jackets and the steel toe capped boots. It would be wise to bring a cushion, a bottle of water, something to eat. Make yourself comfortable. You need to be patient, the earth turns so slowly you have to concentrate to see it. Here – the first shadow’s coming now. See how the world goes dark beneath it? How the next one comes quickly behind? See how they circle, engulfing everything they come across.

The Skirt of the Black Mouth is an artwork – created by Heather and Ivan Morrison – which holds within it another artwork, The Feasting Mouth, designed and built by artist collective Exyzt: a rusted metal sculptural oven, reminiscent of the Easter Island statues and the folded shape of the new building, and a feasting table made from scaffolding.

He bakes bread in the middle of the city. Lights a fire in a rusted oven that looks left over from another time. In the middle of the city, he mixes dough, and waits for the magic of yeast. The oven is like a face – he opens its mouth and it roars orange and red. He bakes bread in the middle of the city.

Sara Muzio from Exyzt explains the collective’s desire to make public spaces that don’t try to flatten out diversity or deny conflict, but create opportunities for people to encounter and negotiate difference. They use fiction and story as starting points, to engage people’s imagination, to avoid ‘architects’ speak’ and to open up new possibilities. The space feels elemental, she tells me. It’s non-urban; it’s coastal, like Dungeness. A lorry drives by and I try to turn the sound of rubber on tarmac into that of waves on a beach. Later, she points at the table in the centre of the space, made from scaffolding planks and poles, ‘The scaffolding escaped,’ she tells me, and I laugh. I like the table even more now it is anarchic and playful, now it has agency and desire.

The Skirt Of The Black Mouth is a beach; the lip of a volcano; a forest. The Feasting Mouth is an oven, a hearth, a volcano, a face. This is a space that embraces metaphor.

I might call a city a forest: see bark, leaves, the breath of the wind in its concrete and glass; know its streets as rivers and its squares as clearings. I might call a city a forest and it might change my mind.

The word metaphor comes from the Greek, meaning to transfer, or carry across. Metaphor is movement. Metaphor is transformation. We appropriate language, describe one thing in the terms of another, and it opens up possibility, creates new connections, allows us to see things differently and therefore to imagine change. I meet with Jason Waddy, a project manager working on the Tate Modern Project, and he talks about the building’s concrete frame as a skeleton, the precast concrete beams as ribs, the concrete panels as skin; all at once the extension is alive – something to care about, something to nurture and build a relationship with. Val Beirne, Bankside Urban Forest Manager, from the local Business Improvement District, Better Bankside, tells me about their masterplan for the area which conceives of Bankside as a forest. ‘The kind of work we’ve done here wouldn’t have happened if we’d called it a park,’ she tells me. A park comes with pre-conceived ideas and fixed boundaries, while there isn’t an edge to a forest, just spaces to move through and the possibility of myth, transformation, transgression.

Words might not be as physical and tangible as a metal oven, a concrete bench, a new building, but they can help transform urban space in subtle and profound ways.

I am a pocket park.
I am the edge of a street.
I am a former building site.
I am a forest glen.
I am a feasting hall.
I am a windswept beach.
I am the skirt of the black mouth.
I am whatever you would like me to be.