I am deep under a hilltop in a train without rails, and my bubbly French guide has a nervous giggle. "Okay," she says. "I am going to give you some safety advice now." And she explains that, while flashguns may be aimed to left or right of the non-existent track, they must never be aimed forward. "This train is guided by laser light," she says, "and the flashes would make the train change direction. Tee-hee-hee."

It is the second time in 24 hours that I have been hauled through a long tunnel. Although on the first occasion I had the entire English Channel above my head, at least I could be reasonably sure that my driver knew the route. Yet these cellars, dug under the edge of Epernay in the 1870s to store wine made by the Mercier champagne house, comprise 18km of dank, criss-crossing passages. And already we have taken three turns to the left and four to the right . . .

There is one consolation, of course. In the event of our train becoming lost, we poor passengers will have access to some 18 million bottles of Mercier's finest. If that doesn't keep our spirits up, nothing will. Tee-hee-hee.

Do you know how they make that stuff? Do you have any idea what's involved in the famous méthode champagnoise ? They don't just pump it up with CO2 out of a gas cylinder, you know. It's not Coca-Cola. It's the real thing. Our girl guide is gallantly rattling through the process right now. "The natural sugar of the must transforms into alcohol and carbon dioxide,"she says. "If you have some questions, you can ask me, of course, after the visit. Tee-hee-hee."

But what she hasn't allowed for is that most of the Brits in this truck know as much about the chemistry of fermentation as Baudelaire knew about Eurotunnel's back-up signalling systems. Fortunately, I've made a little elderberry wine in my time. So move over, Nicole. Papa's going to handle this part of the tour.

The main thing you need to know is that when yeast eats sugar, you're left with alcohol (good), carbon dioxide (good) and dead yeast (bad). To make normal wine, just press the juice out of some grapes, seed it with yeast and wait for the concoction to fizz up like sherbert. Eventually, the gas blows away, the fizzing subsides and the solids all sink to the bottom. At this point, simply pour the clear, alcoholic liquid into a bottle, label it Chateau Dunroamin, and cork it.

But what if the fizzing starts again once the wine is bottled? Argh! Even if the bottle doesn't explode in your face, you'll be left with cloudy wine on account of all that new sediment. And (horror of horrors), the stuff will be fizzy.

And that, for one reason and another, is what used to happen all too frequently in the wine-making district around Epernay. "Cette abominable boisson," is how somebody once described the local product. However did they transform this muck into the classiest drink on the planet?

The name on that little headstone is Moët et Chandon, and it marks a broad strip of vines owned by the celebrated champagne house of that name. Next to it is a field of spuds. Honestly, I know my root crops. They are just plain potatoes. We have left the Mercier cellars now - no point spending one's entire holiday in a tunnel. We have sampled the product (the £3 ticket price included a glass of Brut) and we have crumpled in the face of temptation and made some purchases in the shop.

We have then tripped rather lightly, given the time of morning, into the unexpected brightness of the Avenue de Champagne (funny to think of all those tunnels beneath our feet), and we have noted (for another day) that every other big name in the business seems to have a mini-chateau in this one grand street, along with a museum, a welcoming cellar and, of course, a shop. And we have now driven a few miles out of Epernay to a land where larks sing and distant church bells ring and pinot noir grows alongside potatoes.

Pinot noir? But isn't that a black grape, or at least a red so dark and strong that it gives red Burgundy the feel of blackcurrant jam? What has Pinot Noir got to do with icy sharp, steely-white champagne?

The fact is, this whole vinification business is distressingly complicated. Three sorts of grape go into making champagne (and some cham pagne is pink). One of them, Chardonnay, is a white grape. But although the other two, Pinot Noir and its cousin, Pinot Meunier, are black when grown in Burgundy, in these northerly climes, they rarely get very much colour in their cheeks.

It was a local monk called Dom Pierre Perignon who, some time around 1690, realised that if you pick the grapes good and early, handle them carefully, then press them ever so gently, you have a "white juice" like no other. And this plateau, with its prairie fields of cereal, really is an awfully long way north to be growing grapes. See those low hills? They are the bottom edge of the great chalk rim whose top edge the English know as the South Downs. Here, they are capped with broad-leaf woodland. But below the woodland, on the slopes that rise above the ordinary farmland, is a corduroy fringe resembling nothing so much as an agglomeration of medieval strip fields. These, surprisingly, are the vineyards of Champagne.

Surprisingly, because none of it resembles what I imagined a wine-growing district to look like. They grow sugar beet here and fields of wheat as far as the eye can see. And then there are those spuds, rubbing up against the Pinot Noir. Here, it seems, grapes are just another crop - champagne just another vegetable-processing industry. There is no real magic beneath that gold foil capsule. Or is there?

The vineyards are row upon row of wires, stretched between steel stakes like some vast radio telescope listening intently to space. Captive beneath the mesh are the vines, dormant now, and bent almost double. These plants have been cut back to the knuckle. Only four shoots remain, and these are attached to the framework by little paper and wire twists - the sort you might use to tie a freezer bag. Wires, stakes, vines, shoots, twists. All is precise and neat, measured, calculated, the result of years and years of experimentation and refining. It has the feel of ritual, of secret knowledge and magic.

For how else, other than through magic, did these people turn what was once considered an abominable drink into a byword for celebration?

Consider the problem. Louis XIV is dead, a young man is on the throne and all the world is light and gay. Your gimmicky fizz, now dispensed from new-fangled tough glass bottles with corks tied down by string, seems suddenly just the thing. Indeed, Mme Pompadour herself has mentioned that champagne "is the only wine that leaves a woman beautiful after drinking it".

But there still remains that cloudiness - a product of the second fermentation without which there would be no fizz in the first place. How do you remove the sediment without at the same time releasing all the gas?

It took them another 100 years to work that one out, and many more to perfect the technique. All sorts of ruses were resorted to. Bottles were stored cork down and at an angle of 45 degrees in big wooden racks called pupitres. Once a day for two months, a riddler in some dismal cellar would rotate eachbottle a quarter of a turn, just to persuade the wretched sediment to settle on the inside of the cork. The dearest champagnes are still riddled by hand.

My favourite trick is the freezing. In 1896, someone realised that if you plunge the end of the bottle into a freezing solution, the wine in the neck would form an icy plug that would trap the sediment prior to removal of the cork. But perhaps the champagne houses' greatest achievement of all lay in their early mastery of a brand new and infinitely powerful type of magic - advertising.

No sooner had Eugene Mercier had his vast cellars excavated at Epernay than he hit on a brilliant idea. He would open them to the public. It was the start of something big. At the 1889 Universal Exhibition in Paris, his attention-seeking stunts caused almost as much of a stir as the Eiffel Tower. And at the 1900 exhibition, Mercier joined forces with the Lumièere Brothers to make the very first advertising film.

Bubbly was on the brink of finding a mass market, and the rest, as they say, is tee-hee-hee.

On the grapevine

The English-speaking staff at the Office de Tourisme in Epernay will happily send you away with an armful of leaflets and brochures about the things to do in the region.

There are chIteaux and chocolate factories, farm museums and boat trips - and enough champagne-related visits to keep you busy for several months. Some of these are open all year round, but many are closed at weekends until the beginning of May.

The grape harvest usually starts in late September and, for three weeks, the vineyards are at their most attractive - and their most crowded. It's reckoned that around 60,000 people turn out to help pick the crop (mechanical harvesting is forbidden) and anyone thinking of just turning up and joining in should think again.

A number of vineyards do work in partnership with the tourist office, however, and would-be pickers and treaders should write or telephone for details well in advance.

A more peaceful way to take in the surroundings is to drive into the hills south of Epernay and follow the trail around 12 villages. Each village - these are quiet, ancient places with medieval churches and vine yards that spill into back gardens - has erected a painted sign depicting one of the stages in the tending of the vines. Pick up a map at the Office de Tourisme, 7 Avenue de Champagne, Epernay; tel: (33) 03 26 53 33 00; fax: (33) 03 26 51 95 22.

An ideal companion for a trip to the area is Maggie McNie's book Champagne, published by Faber and Faber at £12.99. McNie gives a detailed yet readable history of the wine, and explains how the topography and geology of the region have influenced the product from Roman times to the present day.

The practicals

Crossing to France through Eurotunnel costs from £189 for a 5-day break. The price includes a car and all its occupants. Promotional tickets must be booked in advance. To book call 08705 353535, or visit www.eurotunnel.com. The drive to Epernay takes between three and four hours. David Newnham stayed at the Hotel Aux Armes de Champagne (03 26 69 30 30 www.aux-armes-de-champagne.com) in L'Epine, a village about 30 mins drive to the east od Epernay, and five minutes from historic Chalons-en Champagne. Rooms cost between £42 and £79 a night, and the luch menu begins at £13. For reasonably priced evening meals (everything from snails to pizza) try Le Vulcano (03 26 68 54 51).