Under, which also means “wonder” in Norwegian, is located on the country’s southern tip, and is submerged in the North Sea.
Designed by Snoehetta, the Norwegian architecture firm behind the September 11 Memorial Museum in New York, the restaurant already has 7,000 customers booked in.
Although doors opened on 20 March to friends and family of the owners, paying guests will gain access from April.
There’s room for around 40 guests in the underwater dining space, which provides ocean views through one giant glass window at the back.
The rest of the design is stark so as not to steal focus from the fish, with wooden walls, ceiling, tables and chairs, concrete staircase and neutral flooring.
This is no casual eatery – the full 18-course tasting menu with drinks costs up to 3,700 krone (£330) per person.
“The fascination is just this movement from above water to underwater through the building,” Snoehetta’s founder Kjetil Traedal Thorsen told Reuters. “The big window exposes the underwater not like an aquarium, it’s the real thing,”
He said the building is built to withstand extreme weather, including “the wave of the century”.
The exterior, a sloping wood and concrete cuboid that slants into the ocean, has a modern aesthetic, and is accessible via a steel bridge.
Under is owned by the two Ubostad brothers who also have a nearby hotel. Gaute Ubostad said they expect to attract around 12,000 diners each year.