Paris’s oldest bridge has begun a dramatic transformation this week, as the artist JR – often dubbed the “French Banksy” – commenced inflating a colossal “cave” over the historic Pont Neuf.

This monumental, rocky illusion is gradually enveloping the 17th-century landmark, which has facilitated crossings over the Seine for Parisians for more than four centuries. By Thursday, the heart of the city appeared to host a rising prehistoric cliff.

The overnight inflation process, which faced initial delays due to adverse weather, marks the most striking phase of a project that has been over a year in the making. This ambitious public artwork, among the most significant Paris has witnessed in decades, has been financed through sales of JR’s work and support from several corporate partners, and is set to open to the public on 6 June.

“We’re about to leave something pretty incredible in the middle of Paris,” JR told The Associated Press earlier this year at his studio in the city’s east, wearing his trademark hat and shades.

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. This time he wanted Parisians to do something unusual on their busiest bridge: stop.

Visitors will be able to walk for free through a long, dark tunnel that lets in no daylight and where, according to JR, people “will lose track of time.”

The numbers are startling. The structure is 120 meters (393 feet) long and 18 meters (59 feet) tall — which is 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. The fabric was hand stitched by 25 artisans in a village in Brittany.

Nothing digs into the historic stone.

Cut the air and the cliff would sink like a held breath — a collapse JR’s engineers spent weeks rehearsing in a hangar at Orly airport to be sure that if the power ever failed, the rock would come down gently.

The artwork, called La Caverne du Pont Neuf, 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.

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

His idea, he said, is to bring “mineral and nature” back to the heart of the city. He is not covering the bridge but undressing it — sending the dressed stone back to the limestone quarries from which Paris itself was cut.

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. Air, JR likes to say, leaves no scar.

Then, like the golden wrapping 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.