Located in the remote Lindesnes area, the 495-square-metre structure is submerged off of a craggy shoreline and now doubles as an artificial reef.
The Norwegian studio designed Under as a concrete tube that is intended to resemble a sunken periscope. The concrete was left exposed externally, forming a rough finish onto which algae and molluscs can latch.
According to Snøhetta, it has now been "embraced by nature" just as intended after completing in 2019.
"The underwater restaurant Under on the Norwegian coast of Lindesnes was designed to fully integrate into its marine environment over time, as the roughness of the concrete shell was aimed to function as an artificial reef, welcoming limpets and kelp to inhabit it," the studio told Dezeen.
"Now, we are all thrilled to see all the planned integrations coming to life, as nature has fully embraced and inhabited the building."
Under was both Europe's first and the "world's largest" underwater restaurant.
Its walls, which are slightly curved and half a metre thick, were engineered to ensure resistance against the waves and water pressure.
As well as providing a unique dining experience for visitors, the building is hoped to attract marine life, boosting the biodiversity of the area and providing fresh food for the chef.
It also provides marine biologists with a unique space to study marine life without disturbing it and encourages visitors to learn about underwater life by observing it up close.
"Many argue that for mankind to truly take better care of our nature, more people need to see and learn more about it," the studio said.
"This also lies as a core idea in the project of Under. Being able to give people greater insight into life in the sea, makes the Under restaurant an extra exciting project for marine biologists."
According to the studio, the site is very biodiverse, as it is situated where the Baltic Sea meets the Atlantic Ocean.
The algae and molluscs that now cling to the restaurant's surface form an artificial mussel reef that will purify the water and naturally attract more sea creatures.
Inside, marine life is framed through a giant acrylic window, which spans 11 by three metres and is visible from each level within the building.
To enable visitors to observe this sea life through the window at night, Snøhetta worked with design studio Light Bureau to create muted seabed and interior lighting.
Since the underwater restaurant's completion, the studio has also added a series of structures to the surrounding seabed, such as rocks, which are hoped to provide an "even better breeding ground" for organisms.
Shortly after it opened, Snøhetta released a video of its construction process that involved flooding the structural shell to make it sink to the ocean floor five metres below.
Snøhetta is an architecture and design studio founded by architects Craig Dykers and Kjetil Trædal Thorsen in 1989.
The photography is by Timon Koch.
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