A giant inflatable mountain has appeared in the heart of Paris

Making public art is not always a walk in the park. Forty years after Christo and Jeanne-Claude wrapped the Pont-Neuf, the oldest bridge in Paris, the foundation that bears their names asked the French photographer and street artist JR to commemorate the event with a creation of his own.

The result, La Caverne du Pont Neuf, has been a Herculean journey, from conception to funding (no public money was spent) to construction. On 16 June, JR announced its opening to the public – 10 days late – with a sigh of relief. '[Christo and Jeanne-Claude's] project at Pont Neuf, like mine, will have been the most difficult of our respective careers,; he said. For his predecessors, the main obstacle had been gaining the necessary authorisations. For JR, it was the vagaries of Mother Nature, which made the project what he calls a 'climate odyssey.'

Whereas the 1985 intervention emphasised the shape of the bridge, JR says he 'wanted to deform it, by making it almost disappear.' The project, 120 metres long and up to 18 metres tall, looks like a snow-capped mountain dropped into the city centre, with a tunnel-like cave inside. Like many of the city's monuments, the Pont Neuf was built using age-old limestone from quarries around Paris. The Caverne references that history, contrasting brute stone (or its idea) with the elegance of the surrounding architecture. Inside and out, the structure is printed with pixellated black and white photos of rock from the quarries.

The Pont Neuf is a protected monument, so there was no question of disturbing the surface in any way. At first, JR's studio considered using scaffolding to build something in a fairly classic way. This evolved into the idea of an inflatable installation, a concept Christo and Jeanne-Claude also explored. It would be lighter, faster to build, and more sustainable. (The fabric will also be reused or recycled after the project comes down.)

More than 800 people took part in the construction: engineers, sound and light technicians, seamstresses, printers, and more. An artisanal company in Brittany specialising in inflatable textile structures designed the fabric in lightweight PVC, sewing the pieces together in an intricate pattern like a puzzle. The final installation is composed of four layers, held in place and separated by pressurised air. In fact, air is the primary element, nearly 20,000 cubic metres of it.

The studio tested the Caverne's individual modules in a hangar at Orly airport before commencing installation on the bridge on 11 May. Weather conditions were extreme from the get-go, starting with a week of cold temperatures and nonstop rain, followed by an unseasonal heat wave. At around midnight on 20 May, the team held their breath as they inflated the structure along with its trompe-l'oeil cover. The next morning, Parisians woke to find a mountain range where a bridge once stood.

But four days before the cave was to open to the public, nature struck again with a freak hailstorm. A fierce gust of wind, intensified by the river and the mountain wall, ripped the printed envelope on either end so that it fluttered like a flag. Parts of the inflated structure also suffered, though the damage was contained by the separate modules.

Now it would be impossible to meet the 6 June opening date. And the closing, 28 June, was set in stone, since the Caverne had to be taken down in time for the national holiday. For two days, the team held crisis meetings, consulted experts, and studied the various possibilities—from dismantling the structure to leaving the exterior white, an option JR refused. Finally, it was decided that the printed fabric would go back to Brittany to be repaired. Another team would remove the sound and light installations that had been inserted between two fabric layers and build scaffolding inside the structure, so that the envelope could be replaced from above. Over four nights, cordistes repositioned the printed fabric onto the installation like Alpine mountaineers, a risky operation that had been purposely avoided during the original installation.

Camille Pajot, the Caverne's art director, compares the lengthy, ambitious installation to an epic open-air performance. 'It was like three projects in one. Attaching the canvas to the bridge itself wasn't easy, with the boats going by. Inflating the mountain was pharaonic. And finally, installing light and sound in the interior. And since it was outdoors, everyone could witness our successes and failures.' But, he admitted, that was part of the beauty, too.

On 15 June at 6 p.m., La Caverne finally opened to the public, secured by ropes on the outside. JR opted to leave black marks where the repairs took place, as a reminder. Inside, a total experience touches all the senses, as visitors pass from daylight into the darkened passageway, then back out to the light again. Thomas Bangalter, a former member of Daft Punk, composed an electronic soundtrack like the howling of wind in the mountains. Sarah Bouasse created an olfactory experience around geosmin, the compound that gives damp earth its scent. The company Snap, one of the project's main sponsors, provided an augmented reality experience, including virtual bats and a dancer.

Days after the opening, another week-long heat wave hit Paris. It was surprisingly temperate inside the cave, and visitors were in no hurry to leave, marvelling at the rock-like effects, waving their arms around to watch the augmented reality in action. According to JR, the installation only came alive once the public took part. 'In the end, the audience is the work. That is the essence of La Caverne: it must be viewed through the lens of storytelling and adventure, rather than as a monumental sculpture.'

La Caverne du Pont Neuf runs to June 28 and is open 24 hours a day. Perrotin art gallery in Paris exhibits JR's preparatory "sketches" of the Caverne until 25 July.