I’m ushered into what looks like an upright glass coffin, told to fold my hands across my chest in the classic corpse position – and then push a green button. I’m wondering exactly what I’ve got myself into when the trapdoor falls open and I plummet into a tube of fast-flowing water. It’s up my nose, in my mouth; I can’t see and can hardly breathe for a few tumultuous seconds before the gradient of the water slide reduces from vertical to merely steep and I’m propelled around more bends and spat out at the bottom. I feel like I’ve been flushed down a toilet.

The Vildfål is one of the more extreme experiences at the new Rulantica indoor water park, which opened on 28 November. Half an hour’s drive north of Freiburg im Breisgau in south-west Germany, it’s next to Europa-Park, the country’s largest theme park. Both are owned by the Mack family, a dynasty of entrepreneurs who have been enticing visitors to this corner of the Black Forest since 1975. In building the €180m Rulantica, the family has made its biggest single investment to date.

The idea for a water park came from Europa-Park customers, according to Thomas Mack, co-managing director. In making the suggestion a reality, the family hopes the theme park’s 5.6 million annual visitors (95% are German, French or Swiss) will want to sample Rulantica, too, and will stay a little longer in one of the resort’s hotels. There are now six of these, the Krønasår having opened next to Rulantica earlier this year. Ticketed separately, the water park also gives customers a reason to visit year-round – while Europa-Park closes for a couple of months in winter, Rulantica will remain open all year (online prices: adults €38.50, 4-11 €35.50, under-4s free).

Twenty years in the planning, construction took 26 months and was completed on time by project manager Charles R Botta. The result is a 32,600 sq m shell-shaped indoor space styled as a mythical island, with different activity areas grouped around a central wave pool, and an additional area outdoors. Instead of the usual water park decor of palm trees and pirate ships, Rulantica is inspired by Scandinavia. Waterfalls trickle over rocks, trolls populate a playground for small children, and a lazy river drifts under stalactites through a cave, where mermaids and fish hang out with Snorri – the park’s octopus mascot – in a projected animation.

It’s an appropriate theme for a park that prides itself on its eco-credentials. Rulantica’s green measures include 3,000 solar modules on the covered car park and a filter system enabling 80% of the wastewater from its pools to be recycled. Steps were taken during construction to protect animal and plant habitats around the park, including creating a bat corridor and a bee pavilion – the resulting honey is available in the Krønasår hotel.

I forget to pick up a map but it’s fun to explore without. I climb a stairway and discover a door to some outdoor rapids. A beached steamship turns out to contain a set of family-friendly slides and a swim-up bar in the Skog Lagune (lagoon) is a pleasant spot to sip a cocktail in a whirlpool bed, despite one couple treating it as snog lagoon.

There are 17 water slides, from entry-level thrills for tots to the full toilet-flush experience. I try out the Isbrekker, two short slides ending with a 1.5-metre drop into a pool, and the Stormvind, where I sit on a raft on which I’m sucked down a tube into a giant whirlpool. My favourite – and many others’, judging by the queue – is the Vinter Rytt, where up to four people cling to an inflatable as it plunges down a vertiginous tunnel before sliding up a sheer wall and back down again, bringing a moment of weightlessness.

The park is designed for relaxation as much as adrenaline. There are deckchairs and loungers, private booths and sofas available at extra cost, and several snack bars and cafes, where purchases are registered on your wristband and paid for on exiting the park. However, the menu of burgers, pizza, pasta and salads is uninspiring and there are no free drinking water fountains. Instead, you can buy a Rulantica-themed keep-cup (€17.50; €16.50 if bought online in advance) and then fill up on free soft drinks from the dispensers. Since the park’s temperature is 32C and accessing the slides involves climbing plenty of steps, hydration is essential.

With its menu of Scandinavian and German dishes (pork schnitzel with potato salad €16.50, Swedish meatballs €15.50, roasted salmon €22), eating in the Bubba Svens restaurant in the neighbouring Krønasår hotel is a classier affair. In keeping with the Nordic theme, the hotel is a collection of brightly coloured wooden buildings set on a fjord. Inside it’s styled to resemble a natural history museum. A giant skeleton of a mythical sea serpent dominates the foyer and display cabinets feature Nordic outfits and accoutrements from bygone days. It’s all intended to evoke the “story” of Rulantica, a fictional narrative about a lost island, which has even spawned a children’s novel (only available in German).

It’s a pleasing construct with plenty of family appeal but it’s not the story that sticks in my mind afterwards: it’s the thrill of the unknown at the top of a slide, the adrenaline rush of being flushed feet-first down a tube, and the relief of surviving to tell the tale.

• Train travel, Rulantica park entry and accommodation, at the Krønasår (doubles from €225), were provided by Europa-Park. As an alternative to driving, take the train (frequent direct services from Paris, Basel, Freiburg and Strasbourg) to Ringsheim station, 3.5km from the park, and then the 7231 bus

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