Want to taste the world's best (and rarest) beer? Then you'd better get in line early.
It was a quest for the holy grail of beers, the legendary brews of the remote Belgian Trappist abbey of Saint Sixtus. Searching the west Flanders platteland, we drove through countryside so flat the mountains of harvested sugar beets provided the only topographic relief. A dome of low, gray clouds hung overhead as we bumbled down farm lanes and allées, seemingly forever thwarted.

With more than 500 different kinds of beers, Belgians enjoy one of the world's great brewing traditions. The country's 100-plus breweries produce an array of styles: wild-yeasted lambics and cherry-tinged krieks, Belgian pale ales and oud bruin brown ones, lively witbiers, Flanders' crisp golden Duvel (named after the devil), Wallonia's intriguing and unpredictable saisons.

But above all these beers, it's the ales of the six Belgian Trappist monasteries that have captivated beer aficionados worldwide.­ And these connoisseurs have canonized the smallest and most isolated monastery of the bunch, Saint Sixtus, as the finest Trappist brewery of all, hailing the monks' rich, impossibly complex Westvleteren 12 as the best - and rarest - beer on earth.

The Spartans of the monastery movement, Trappists adhere to an austere cloistered regimen that requires self-sufficiency and manual labor. They founded Saint ­Sixtus in 1831, and by 1836 they were brewing beer that garnered renown. But in 1945, the monks made a momentous decision: They would no longer sell commercially. To focus on spiritual matters, the monks decided to limit their sales to the abbey and the nearby In de Vrede café, which remain the only authorized sources of Westvleteren beer. "[It's] the most famous beer in Belgium," says Michael Jackson, famous British beer maven and author of The Great Beers of Belgium.