There’s something fishy going on here. I’m standing in the Lost Chambers of Atlantis, staring at what, I have been gravely assured, are the submerged ruins of Plato’s ancient, doomed civilisation.

The last time I saw daylight, however, I could have sworn I was on the Palm, an island off the coast of Dubai that I don’t think Plato had heard of, being as they started building it only seven years ago. You don’t have to be a Time Team presenter to know it doesn’t quite add up.

The Lost Chambers are the star attraction of the Atlantis, a new, 1,539-room mega-resort that will open to the public on September 24. Last week, I was the first British journalist to take a look around the near-finished article, and I was gobsmacked. It’s one of the most impressive and ambitious resorts I’ve seen. It’s certainly the most ludicrous.

So ludicrous, in fact, it’s almost heroic. It takes a certain damn-the-torpedoes guts to spend £750m on a premise this self-evidently daft: the “discovery” of a 10,000-year-old civilisation that never existed, on an island that’s still being finished. But we’ll get to that in a minute. For now, the impressive stuff.

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You approach Atlantis up the “trunk” of the Palm Jumeirah, as it’s formally known. In sheer engineering terms, it’s a boggling thing. Where there was nothing but sea five years ago, they’ve built a three-mile-long island with fronds radiating from the centre. Right at the crest, in prime position, the 395ft towers of Atlantis emerge slowly through the heat haze.

From the outside, the architecture is a bit odd. It’s supposed to look “Atlantean”, which seems to mean a lot of fish motifs, but they couldn’t resist throwing in a few other elements: they’ve ended up with Peter Jackson fantasy meets arabesque meets Hilton high-rise, all painted a slightly queasy frozen-prawn pink. I’m not sure it’s what Plato had in mind.

Go in and it gets odder still. The vast lobby is dominated by Dale Chihuly’s 35ft-high glass sculpture, which looks like cascading multicoloured spaghetti. There are garish “mythical” murals, and they’ve covered a good deal of the acres of floor with a turquoise-and-yellow swirly carpet – sea and shells, I think, though it’s hard to tell.

Step off that carpet and you’re in the serene and genuinely stylish spa, or David Rockwell’s sensational bamboo and wood design for the Nobu restaurant. It’s like that all over. The avenues and halls go on and on, mid1980s Dallas styling around this corner, cutting-edge contemporary around that – the most expensive design identity crisis in history.

The food is as ambitious as the rest of it. There are 17 places to eat: Giorgio Locatelli, the best Italian chef in London, has a trattoria here, and they’ve drafted in Michel Rostang from Paris and Santi Santamaria from Spain. That’s seven Michelin stars right there.

What about the rooms? The standard ones are a good size, high-spec and pretty bland, which is something of a relief. For more drama, you can always go for the Lost Chambers suites: the bedrooms look out through huge underwater picture windows into the resort’s 11m-litre lagoon, stocked with sharks, rays,angel-fish, trevallies and more, in dense, multicoloured shoals.

Fine for romantics, as long as you don’t mind a fishy audience – though the sight of the rays gliding past is so mesmerising, you might not get round to anything energetic.

If money’s no object, you’ll want the Bridge Suite, which spans the archway between the two towers. A British family are the first bookers, paying £45,000 for three nights: for that, they get three bedrooms, four staff and a gold-leafed dining table seating 18. Not the food to go on it, though – that price is B&B.

Back down to earth, the beach is fine, though don’t expect much from the scenery. It faces back to Palm island, which may look great on a map, but is surprisingly ugly close up, with its densely packed, colourless villas and miles of strangely arid, unwelcoming beachfront. Nature does islands rather better than man.

Still, you get free access to Atlantis’s 42-acre Aquaventure waterpark. It’s a cracker, with a 1½mile river to float in, a fantastic children’s playground and cutting-edge rides topped off by the Leap of Faith, a near-vertical 90ft slide that shoots you through a shark-filled lagoon like a bullet out of a gun.

There’s buckets more here: two kids’ clubs, a nightclub, posh shops (Tiffany, Graff, Cartier); oh, yes, and a dolphin “conservation centre”. Yeah, right.

The mammals were caught in the Solomon Islands and shipped here to live in tanks so we could pay to swim with them. I didn’t.

The keynote attraction, however, is the Lost Chambers. In a dimly lit stone labyrinth full of startled fish are great bits of fallen masonry covered with mysterious runes (though, presumably, they’re not that mysterious to the guy who made them up). You wouldn’t think you’re supposed to take all this stuff seriously, but they do, they really do.

From the top down, Atlantis’s staff treat their newly constructed ruins with po-faced reverence. Their eyes take on a spooky, glazed look when they talk about it, like freshly indoctrinated members of a Californian UFO cult.

“This is the Abyss,” my guide says. “It was here the Atlanteans mined their minerals – they lowered their miners down this well. Fascinating, isn’t it?”

“But... it’s not real, is it?” I mumble. My words simply don’t register. “We expect a lot of school parties,” he says. “Education is a big part of our work.”

Schools? Education? They’re kidding, aren’t they? Yes, kids will love Atlantis, and yes, it’s certainly worth seeing – a phenomenon, a bonkers colossus – but, really, a few days will do it. Any longer and you might end up getting that spooky-eyed look yourself.

Kuoni (01306 747002, www.kuoni.co.uk) has three nights at Atlantis in October from £879pp, B&B. That’s good value, but expect prices to rocket in winter – Destinology (0800 072 2227, www.destinology.co.uk) has seven nights in February for £3,829pp, B&B. Both prices include flights from London

ATLANTIS IN BIG NUMBERS

The cost: £750m

The size: 114 acres – or 64 Wembley football pitches

The rooms: 1,539, with prices starting at £228 per night for a standard double and rising to £15,000 for the Bridge Suite

The water: 60m litres, including the rides and aquariums – enough to fill 24 Olympic-size pools

The rides: 8, including the 1½mile river ride

The restaurants: 17, three from Michelin-starred chefs

The fish: 65,000 specimens, twice as many as the London Aquarium

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