The zoo opened on 13 June 1996 with two domes. The African dome covers 500 square metres and rises twelve metres; the Asian dome is larger at 700 square metres and fourteen metres high. Then, in 2003, came the one that changed the scale of the place. The South America dome encloses 2,000 square metres under a roof twenty-five metres up, with a fifteen-metre waterfall inside it - big enough that the canopy has real vertical structure rather than the potted-palm compromise most indoor tropical houses settle for. Add the three together and the covered area runs to roughly 3,600 square metres. A saltwater aquarium followed in 2005.

Climate control is the whole engineering problem, and it is handled by computer for a specific reason: the swing in Danish daylight hours is enormous compared with anything a tropical species has evolved to expect. Get it wrong and you disrupt breeding cycles, feeding behaviour, plant growth, and moulting - the entire biological calendar of everything under the glass. So the domes run on their own imported year. Temperature and humidity are held where the biome demands rather than where Jutland suggests, which in February means walking out of a grey Danish afternoon into air thick enough to fog your glasses in about four seconds.

The defining decision at Randers Regnskov is that most of the animals live freely in the domes rather than behind barriers. You are inside the enclosure. Lemurs occupy the African dome; the Asian dome holds pileated gibbons and Komodo dragons; South America has tapirs, manatees, callitrichid monkeys and macaws. There is a snakeyard stocked with free-ranging harmless species, a substantial reptile collection, and the saltwater aquarium with its tropical fish. Among the longer-term residents is Henriette, an albino tiger python measuring four and a half metres - the kind of animal that makes a corridor feel narrower than it did on the way in.

In 2015 the zoo opened an enclosure for jaguars, notable because it was the first exhibit sited outside the domes rather than within them - a deliberate step beyond the original three-biome concept. Alongside it sits Danmarksparken, or Denmark Park, with farm animals and playgrounds, which functions as the deliberate counterweight to everything else on the site: after the manatees and the macaws, a pig. It is a reminder that this is a Danish institution as much as a tropical one, and that most of its visitors arrive with children who will happily spend as long with a goat as with a Komodo dragon.

The numbers are worth spelling out because they explain why a provincial Jutland city is on anybody's itinerary. Roughly 275 animal species live here - about 200 of them in the domes and another 75 in the saltwater aquarium - alongside some 350 species of plant, which is what makes the rainforest read as a rainforest rather than a warm room with animals in it. The complex is described as the largest artificial rainforest in Northern Europe. In 2012 it recorded 314,643 visitors, comfortably more than four times the population of the city it stands in. Nothing about this location suggests a tropical zoo. That is more or less the point.