PARIS (AP) — The oldest bridge in Paris looked Thursday as if it had been swallowed by a mountain.

The transformation is the work of JR, the street artist known as the “French Banksy,” who this week began inflating a giant artificial “cave” over the Pont Neuf, turning the 17th-century bridge that has carried Parisians across the Seine for more than 400 years into a rocky illusion rising over the river.

JR has said the idea of La Caverne du Pont Neuf, is to bring “mineral and nature” back to the heart of the city. He says he is not covering the bridge so much as revealing the stone taken from limestone quarries from which Paris itself was cut.

A jagged mass of gray rock now seems to rise over its arches. From downstream, the landmark appears to have vanished beneath a prehistoric cliff, its stone openings transformed into dark cave mouths above the water.

“I thought, ‘Where has the bridge gone?’” said Marie Leclerc, 62, who stopped on the quay on her way to work. “It’s strange because you know it’s fabric and air, but from here it really looks like stone. Paris feels suddenly ancient again.”

The inflation, carried out overnight after being delayed by bad weather, is the most dramatic stage yet of a project more than a year in the making.

“It’s a gigantic puzzle that has just been finished,” JR told The Associated Press at the bridge as his team prepared to pump in the air. “We’re going to send air inside, and all these rocks will rise into the Paris sky, almost 18 meters high. Once they’re inflated, they stay.”

One of the most ambitious public artworks Paris has seen in decades — funded by the sale of JR’s work and a handful of corporate partners — it does not open to the public until June 6.

The transformation has been documented by the AP since March with time-lapse cameras, including one fixed on a rooftop terrace high above the river, watching the bridge slowly disappear day by day.

From the outside, the installation looks like a rocky mass that “literally” breaks the landscape, said JR, who is famous for pasting enormous photographs on buildings, walls and rooftops around the world. He is often compared to the British street artist Banksy for the style of his work.

“Usually everyone crosses here without looking,” said Julien Moreau, 34, taking photos from near the Seine River. “This morning everyone was standing still. That’s already the artwork.”

JR said he wanted Parisians to do something unusual on their most famous bridge: stop.

“We’re all a bit stressed. We want it to work,” he said, as workers in harnesses readied the structure. “But that’s the beauty of a project like this — its fragility, the fact of working in the street, exposing yourself to everyone.”

Some passersby, he added, “will walk by without even realizing it’s rising. Others will be completely amazed.”

The structure is 120 meters (393 feet) long and 18 meters (59 feet) tall — as high as a six-story building.

Yet it is built almost entirely from air — 80 fabric arches filled with 20,000 cubic meters of it — and weighs only about five tons.

JR’s engineers spent weeks testing the structure in a hangar at Orly airport, simulating a cut to the air supply to be sure the inflatable rock would hold its shape.

The fabric was hand-stitched by 25 artisans in a village in Brittany.

Visitors will be able to walk for free through a long, dark tunnel that lets in no daylight. “You enter into the darkness,” JR said, “and emerge into the light on the other side.”

He described it as a journey each person is free to take in their own way: “Many people will pass through this cave and let their imagination dictate what they feel.”

The artwork is a tribute to a Parisian artistic legend.

In 1985, artist Christo and his wife, Jeanne-Claude, wrapped the same bridge in pale golden fabric — 13 kilometers of rope, a decade of arguing with city hall, three million visitors in two weeks.

The act helped invent the idea of monumental art in modern cities. A square beside the bridge now carries their names — and it is from there that visitors will step into the dark.

“It’s pretty hard to go after them,” JR said.

The cave is also a warning. JR built it as a nod to Plato’s allegory, in which prisoners mistake shadows on a wall for the real world.

“What are our caves today? Our phones,” he said. “Because we believe that our algorithm on social media is the reality.”

Then he walks straight into the contradiction: to enter his cave about screens, visitors raise their phones.

The tech company Snap has built an augmented-reality layer that shows what the eye cannot.

The sound is a low, mineral hum from Thomas Bangalter, formerly of Daft Punk — who was 10 the year Christo wrapped the bridge.

The cave will be open around the clock from June 6-28, closing the bridge to traffic and visible from the quays, from passing boats, even from the top of the Eiffel Tower.

It will coincide with Paris Fashion Week, World Music Day and the all-night Nuit Blanche arts festival.

When it comes down, the fabric will be reused or recycled.

Then, like the golden wrapping over 40 years before, the cave will be gone — and the Pont Neuf, older than the republic and older than the revolution, will reappear exactly as it was.

___

Associated Press journalist Oleg Cetinic contributed from Paris.