The Kattegat is not a glamorous body of water. It is shallow, frequently the colour of wet slate, wedged between Jutland and Sweden, and it has never photographed like the tropics. When Kattegatcentret opened in 1993, that was precisely the argument. Its largest tank, Oceanariet, holds a million and a half litres and stages a scene from the Kattegat itself — the herring that built fortunes along these coasts, Atlantic wolffish with their unnerving dentition, grey seals and harbour seals rolling past the glass. Around 250 species live in the building altogether, and the local ones are given the same volume of water and the same seriousness as anything shipped in from warmer latitudes. Visitors can dive in Oceanariet too. It is a strange and effective piece of persuasion: the sea you drove past on the way here, taken as seriously as an aquarium normally takes the Pacific.

Hajtanken — simply, the shark tank — is what most children have come for. It holds 550,000 litres of tropical saltwater and is threaded by a tunnel, so that the animals pass overhead rather than in front of you: sand tiger sharks, whitetip reef sharks and nurse sharks, sharing the volume with rays. The centre leans into this. Its overnight programme, Zzzzov med hajerne, puts schoolchildren in sleeping bags beside the glass, and roughly twelve thousand pupils a year come through the education service in one form or another. There is a calculation behind the theatre. A child who has slept under a shark is unlikely to grow up thinking of one as a monster, which is the whole point.

In 2022 the aquarium added something that has nothing to do with either the Kattegat or the tropics: a two-storey penguin enclosure, kept at seven degrees Celsius, where gentoo penguins can be watched from close range by visitors willing to stand in the cold with them. It is a deliberate break in the building's logic. Everything else here argues for the sea at the end of the road; the penguins argue for the far southern ocean, and for the fact that a regional aquarium in a town of some fourteen thousand people still has to compete for a family's Saturday.

The tanks are not an abstraction. Immediately off Grenaa the seabed is limestone — the same Danian bryozoan chalk that stands up as cliffs at Sangstrup and Karlby a short way north — and it continues underwater as a reef called Kalkgrunden, rising to within about 1.2 metres of the surface near the harbour. Divers who go out into the real Kattegat here find outcrops, ledges and crevasses running through forests of kelp, which is a considerably more three-dimensional landscape than the flat sandy Denmark of the popular imagination. The aquarium's Kattegat tank is a scale model of something a visitor can, weather permitting, go and swim over the same afternoon.

The stated mission is unfashionably plain — to pass on knowledge about the sea so that more people can enjoy, understand and guard it. The institution has grown into that sentence rather than out of it. The buildings were designed by the Aarhus architects Kjær & Richter. Princess Marie has been the centre's patron since 2013. Some 1,800 square metres of solar panels, paired with a flow battery, now run against the enormous electrical appetite of pumps and chillers, which is a quietly pointed thing for a marine conservation centre to do. In 2022 the centre logged 171,694 visitors. For a building on a peninsula that most Danes drive past on the way to somewhere else, that is a considerable number of people persuaded to look at a herring.