In the depths of Lake Geneva, a team of divers is building an underwater castle. This unusual operation takes place on a sunny Sunday near Geneva, Switzerland. The future inhabitants of this submerged shelter: corals.

Welcome to Rrreefs, a Zurich-based association founded in October 2020 that designs artificial coral reefs made of clay using a 3D printer. An ecological project at the confluence of art, science and new technologies.

Stacked on the platform, the sculptures look like small dungeons waiting to be sent to the bottom of the water. Ochre-coloured, with a ribbed relief, they are tender under the palm, despite their seven kilos. Their shape has been designed to collect coral larvae carried by ocean currents. Thus encrusted, these small animals can develop the hard skeleton that eventually forms a natural reef.

The clay bricks designed by the Rrreefs association, intended to form an artificial coral reef. Their new-generation version was tested on September 10, 2023 in Lake Geneva. © Pauline Grand d'Esnon, France 24

They may represent a modest portion of the seabed, but 25% of life underwater depends on these fragile structures. Their benefits are manifold: corals serve as a refuge for fish, a breeding ground, a source of food, and protect the coasts from erosion.

A real estate agency for marine wildlife

A jewel of life now threatened by global warming and human activity (overfishing, water pollution, marine heat waves, etc.), coral mountains are disintegrating. Half have died in the last 40 years. "Under the effect of stress, corals expel the symbiotic algae, which allows them to feed, and starve," explains Marie Griesmar, co-founder of Rrrefs, wearing a cap, curly hair, and fish on her T-shirt. She helps Hanna Kuhfuss, the other co-founder, who is shackled by her wetsuit, to pull her out of the water.

Rrrefs, which does not pretend to put an end to this desperate process, has made it its mission to provide this shelter for the surviving larvae and a second chance to grow and welcome other living organisms. "I'm a real estate agent for private animals," says Marie Griesmar with a smile.

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"What I like about our project is that it uses the passive restoration method," says Hanna Kuhfuss, a marine biologist by training. Other coral preservation systems involve cloning, but if one of the organisms is sick, it affects them all. Our technique allows nature to act, promoting the development of the offspring of corals best adapted to global warming. Based on natural reproduction, we maintain their resistance."

Four complementary talents

At the origins of the Rrreefs project, we find the singular combination of four talents. It all started at the ETH Zurich, where art student Marie Griesmar was thinking about how to combine her two passions: art-making and diving. She then crossed paths with Ulrike Pfreundt, a scientist specialising in the preservation of tropical ecosystems, who was carrying out her final thesis project on the effects of currents on artificial structures.

From this meeting was born a common dream in the service of the preservation of the ocean. Joined by Josephine Graf, who supports Ulrike in the development of the association and the search for customers, and by Hanna Kuhfuss, the marine biologist, they created Rrreefs at the end of 2020.

The first attempts have been encouragingly successful. Their first test, launched in the Maldives with 100 clay bricks of various shapes, began to flourish. "These larvae settle, and the moment they do, this system attracts a whole community: spores, fish," Hanna says. " And a balanced ecosystem develops, where sea urchins eat algae, etc. Within three months, we had almost as many fish as a natural reef!"

One crowdfunding later, Rrreefs launched its first full prototype, consisting of 228 bricks, in Colombia, in partnership with local scientists. "The teams on the ground call it El Castillo! (Editor's note: the castle, in Spanish)," Marie said proudly.

The prototype created by the Rrreefs association, photographed a year after its installation, in October 2022, is already being taken over by corals and marine fauna. © Aldahir Cervantes

The aim of today's operation is not to attract corals, which live quite far from Swiss lakes. But to test in real conditions their new creations, new generation bricks, larger and heavier, with a view to a new installation in the Philippines, for which Rrreefs has just obtained the green light.

Nothing has been left to chance in the design of the bricks: porosity, shape and colour are the result of three years of testing. "We chose a natural colour, which imitates purple-red algae. It's a visual indicator of a healthy substrate," says Griesmar. The bricks fit on top of each other, thanks to a protrusion on each side, similar to a small chimney. Like a big children's game, all you have to do is put them together.

"To make an impact, you need money"

Under the lake, it's bustling. Part of the team plants anchors at the bottom of the lake to set up the platforms that host the reefs. On the surface, volunteers rope one brick after another through the lake. At a depth of a few meters, a diver receives them, sets them up on a platform, and brings them to the reef assembly site.

However, real-world testing is full of surprises. "We can't see anything underneath, we've lost our way! It took us twenty minutes to find the others," says Mauro Bischoff, the latest member of Rrrefs' permanent team, as he takes off his diving mask.

Activity in the lake – swimmers, divers pounding the ground to set anchors – has blurred visibility underwater. It's time for System D: the team unwinds a long red thread, from the platform to the marker buoy, so that divers can find their way from the bottom. "There's always something you don't plan for," Marie jokes. We have to be creative!"

The team, with an average age of just 30 years and mostly Swiss nationals, converses in English, German or French. Leaning over a watertight black notebook, whose sketches accompany them underwater, Marie and Mauro examine a miniaturized version of their underwater castle.


Marie Griesmar, co-founder of the Rrreefs association, immersed next to their prototype installed off the island of San Andrés, Colombia, in September 2021. © Leila Tazi

With a mullet cut, a tribal tattoo on the back of his neck and a twinkling eye, Mauro is also an art student. He met Marie at the Zurich Institute of Technology, and dedicated his graduation project to designing an improved version of the Rrreefs structures. Around them, a handful of volunteers support the small team to transport the bricks, film their exploits, solve problems.

Full-scale tests, calls for donations, obtaining prizes, recruiting customers: today, the association is at a crossroads and is about to become a company, the only way, according to its founders, to reap the profit necessary for its bubbling ambitions.

"We're going to keep the association to do research, but to have an impact, you need money," says Marie. In the long term, the co-founders, who collectively decide on all the developments of their projects, imagine partnerships with hotel chains. "If we can raise awareness among tourists, make them discover this project."

A Belgian couple stops to admire the miniature reef. Marie pauses in her preparations to tell the story of Rrreefs, once again. She sums it up: "This project is not just about doing a good deed. It comes from the heart."

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