The brightness vanished the minute I opened a creaky wooden door on rue Gheude and entered the Brasserie Cantillon, near the Gare du Midi, a century-old brewery producing beer so ancient, so idiosyncratic, that it is often spoken of in the reverent tones reserved for grand cru bordeaux and the rarest single malts.

The beer is called lambic, and the traditional sort has anappetizing dryness, an invigorating tartness, and a complexity that rivals the finest sherry's; it's a beer boasting aromas suggesting everything from lush fruit to mineral earth; a beer that makes you suspect that everything else you know about beer is a lie. Great lambic is the brewing world's Holy Grail, Cantillon its most sacred temple.

A rarity today, lambic was once the defining drink of the Pajottenland, a rich agricultural region southwest of Brussels on the Senne River. Cast an eye on any of Brueghel's famous depictions of Flemish celebrations, and you'll spy jugs of what is believed to be the peasants' notorious "yellow beer" being consumed with great relish. In countryside cafes, you can still find the locals—many of whom look as if they'd stepped out of themasterpiece known as Peasant Wedding Feast—enjoying lambic poured from rough-hewn pitchers alongside plates of mussels, radishes, herbed cheese, and tete pressee (head cheese).

Likewise, parts of the world's most famous lambic brewery appear unchanged from Brueghel's time. At Cantillon, as at all traditional lambic breweries, scant attention is paid to the rules of modern beer making. Whereas other beers are fermented with carefully controlled yeast strains, lambics owe their fermentation to a wildparty of airborne microflora that includes more than 100 identified yeast strains and 50 kinds of bacteria. Since virtually everything in the brewery is thought to have the microbiotic potential to affect this spontaneous fermentation, there is a certain endearing grubbiness to Cantillon. The air inside the brewery makes for an olfactory adventure, perfumed as it is with a musky potpourri of damp wood, wet grain, and a heady mix of barnyard aromas known collectively and affectionately as "horse blanket".

But lambic's unique microbiotic mix provides only part of the great beer's character. The winey, aggressively citric flavor of traditional lambic is also influenced by its years of aging in wooden barrels, some of them decades old, arranged in shadowy racks. Astringent notes are added through the use of a large percentage of unmalted wheat, along with the more typical malted barley. And in the case of the famed lambic called gueuze—produced by the methode champenoise-like blending and bottle-refermenting of lambics at least one and up to three years old—the aging process plays a vital role in giving the beer an enormous complexity that makes it quite unlike any other.

Lambic's fiercest advocate is Cantillon's Jean-Pierre Van Roy. An affable grandfather with the physique of a man half his age, Van Roy is utterly unwavering in his commitment to traditional lambic. He refuses to sweeten his gueuze in order to tone down its natural tartness, as is the practice at many commercially oriented breweries. In a similarly purist vein, he uses only whole fruit in his fruit-flavored lambics—never juices or syrups. In Cantillon's fruit beers, pounds of cherries, raspberries, apricots, and even grapes are added to the already two-year-old barrels of lambic to macerate for as long as ten months. Then the renewed fermentation continues for at least three more months—sometimes even for several years. Van Roy declines to join the lambic brewers' association because other members make sweetened beers in addition to traditional lambics. "Those other beers, they are not lambics," hesays with a shrug. "I would like them to make an association of only producers of real lambics, and I would join that, but I cannot join this group." The "other beers" Van Roy speaks of so dismissively are what most people think of when they hear "lambic". Sweetened with sugar or fruit juice and profoundly fruity in flavor, those brews, from brewers like Lindemans and Chapeau, have little in common with, say, the musty-dry, tart kriek (cherry) and framboise (raspberry) created by traditional methods.

Real or not, the sweetened lambics help pay the bills nearby in Vlezenbeek at Lindemans, at least according to Roger Mussche, one of the brewing world's leading microbiologists and a close friend of and consultant to the Lindemans family. Mussche does not hesitate to say that Lindemans's sole traditional lambic, a firm and flavorful gueuze called Cuvee Rene, is his favorite of the brewery's beers. But it is the fruit beers—young lambics blended with 25 to 28 percent pure fruit juice—that make up the overwhelming majority of this country brewery's annual sales of more than 10 million bottles of beer. "If we could ask the same price as Champagne, we'd start producing all traditional lambics tomorrow," says Mussche. "But at over 32,000 hectoliters [of production], the capital and space requirements would make it impossible any other way."

For that reason, most traditional lambic producers run small operations, with outputs measured in the hundreds rather than the thousands of hectoliters. Some, like Hanssens Artisanaal, of Dworp, don't even brew the beer themselves. Instead, they buy it from others (the day it's made) and age it on the premises.

The practice of aging and blending lambics, requiring skills similar to those of an expert whisky blender, was once commonplace across the Pajottenland. For centuries, cafe owners and beer distributors bought lambic from the brewers after it had been inoculated by the wild yeasts but before fermentation truly took hold, then fermented and aged the beer in their own barrels in their own cellars. Since so much of a lambic's character comes from aging and blending, those beers are considered to be creations of the cellar rather than of the brewing process.

Hanssens Artisanaal, only recently handed down from Jean Hanssens to his daughter Sidy and her husband, John Matthys, is one of the few blenders left in Belgium. Like the beers of Cantillon, the Hanssens lambics are the stuff of legend, particularly in the United States, where traditional lambics have won a small but growing contingent of aficionados. One Hanssens beer of particular note is the kriek, which exemplifies how lambics fermented with whole fruit differ from those flavored with juices. Rather than the sweetness of raw juice, the Hanssens beer displays the complexity of fruit skins, flesh, and pits that comes from the cherries' having been added whole to the barrel and fermented down to a pile of bare stones. This more intense fruitiness complements rather than overwhelms the dry, tart character of the lambic and makes the beer well suited to main-course dishes based on beef (and on horsemeat, much appreciated in Belgium), not just sweet desserts.

Ten minutes from Hanssens, in the village of Beersel, Armand Debelder grins when discussing his desire to make the transitionfrom blender to brewer. A former chef whose family has been producing lambics for two generations, Debelder, along with his brother Guido, inherited the 3 Fonteinen restaurant and its lambic cellars in the 1980s. Until recently the only place to sample the house lambics (the gueuze is now sold elsewhere in Belgium), the restaurant has long attracted beer enthusiasts from around the world.

Debelder's love of traditional lambic pushed him to take the next step. After years of discussion, he and his brother have nowseparated the company into two businesses, Guido assuming control of the restaurant and Armand taking charge of the beer. Incooperation with the blender Van Vereweghe, located in nearby Gooik, Armand has installed a brewery at 3 Fonteinen.

The going has not been easy, Debelder says, and he is unsure whether his brewery can survive on the production of traditional lambic beers. He cites the difficulty of selling tart, complex beers in a world of sweet, simple lagers but reiterates that he's determined to do it anyway.

As I leave 3 Fonteinen, Debelder stares deep into my eyes, his face a portrait of pure emotion. "It is my one hope that this beautiful thing will survive," he says. "Because it would be terrible if future generations were not able to experience this magnificent beer."