World-class scuba diving has not, historically, been among Belgium's claim to fame.

Chocolates, the Smurfs, the European Parliament and statues of micturating minors, yes. Clear blue water and coral reefs, no.

So 11 years ago, a civil engineer with a passion for the underwater world decided that it was time this situation was redressed. John Beernaerts's dream of creating the world's first indoor diving complex began as a simple doodle, sketched on a napkin in a Brussels bar.

Today John and I are sitting at another bar as waiters carry drinks and Thai food to tables. Facing us is a row of large, square windows. Every few seconds a diver drifts past, blowing bubbles in the blue space behind the glass. It's a human aquarium.

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“It was huge work to transform this little design into reality,” Beernaerts, 47, says. “It was an obsession. I was at the A and I had to make it to Z.”

Seven years, £4 million and four girlfriends later, he opened Nemo33 on the outskirts of Brussels. It was an immediate success.

At 33m (105ft) deep, Nemo33 is the world's deepest swimming pool. And its 2.5 million litres of spring water are kept at a constant, you've guessed ... 33C (91F). “The idea was to make tropical water,” Beernaerts says. “To bring the Bahamas into Brussels.”

But it's much more than a posh swimming pool. Nemo33 is purpose-built for scuba diving, with every design feature informed by Beernaerts' 32 years as a diver and 20 as an instructor.

After kitting up by the poolside - all equipment is provided - he takes me on an underwater tour. Dropping down to 10m, I pause at the observation windows to wave to my wife and son in the restaurant, and then we make our way to the 33m pit.

We're greeted by a wall of bubbles rising from the divers below. Descending through the jumble of fins, arms and legs - the pit can get crowded at weekends - we find ourselves alone at the bottom. I can feel the pressure of the water bearing down.

After a few minutes spent staring up at the distant disc of light visible high above, it's time to ascend. Beernaerts shows me the underwater tunnels and chambers, filled with breathable air to allow instructors to talk to students without surfacing.

More than 100,000 divers, from all over Europe, have visited Nemo since it opened in 2004. “There are people who have been coming once a week from the beginning.” he says. “It is like a sauna or yoga. All the stress of work is gone in one hour.”

As a scuba training and practice facility, Nemo33 is in a class of its own. Every city should have one. It's safe, comfortable and convenient if you happen to live in Brussels.

But is it worth the journey from the UK? Nemo33, a ten-minute taxi drive from Brussels' Gare du Midi Eurostar terminal, is possible as a day trip, but it makes more sense as part of a family weekend break. Just turn up with your scuba certification and swimming gear and enjoy a relaxing 45 minutes underwater.

Afterwards there is plenty of time to venture outdoors and immerse yourself in Brussels; its cafés and restaurants, the recently renovated Atomium, the Museum of Natural Science's dinosaur gallery and the city's famous chocolatiers. After all, you can't buy chocolate Smurfs in the Bahamas.

NEED TO KNOW

Nemo33 (0032 2 332 3334, www.nemo33.com) charges Euros 22 per dive. Eurostar (0870 5186186, www.eurostar.com) has return fares to Brussels from £59.

Hotel Amigo (0800 988 4040, www.roccoforte collection.com) has rooms from £158.

Further information: Belgian Tourist Office (020-7531 0390, www.belgiumtheplaceto.be)

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