There is no sign at the Vega Sicilia winery and no tasting room, and anyone uninvited who knocks at the door will be turned away. Waving a wad of money to buy a bottle of wine will do you no good; nothing at the winery is for sale.

Vega Sicilia is one of the most highly regarded—and unusual—wines on the planet. Of its two top labels, called Unico and Unico Especial, the first is released only after aging for at least a decade in barrels and bottles, compared with three years for the best wines from Bordeaux. The second, Especial, is a blend of grapes harvested in three different years, a rarity in the world of red wine, so the bottle doesn’t have a vintage date on it. And if Vega Sicilia’s winemaker is dissatisfied with the quality of the harvest in any year, the winery will refuse to produce a single bottle of Unico. This has happened four times in the last two decades: 1992, 1993, 1997 and 2001.

Fewer than 7,000 cases of Unico are produced each year on average, and only about 10% of those go to the U.S. The current release of Vega Sicilia Unico, from 1999, sells for between $300 and $400 in the fewer than 100 U.S. wine stores that carry it. Procuring it in Spain is also a challenge: Individuals and even retailers and restaurants in Spain must add their names to a 5,000-person waiting list to become eligible for an annual allotment of the wine. If you do score a bottle, you may want to wait around 20 to 30 years to drink it, because it will take that long to reach its peak thanks to its long aging time in barrels.

The 146-year-old winery, in the Ribera del Duero wine region of Spain, is a two-hour drive north of Madrid. Since 1982, it has been owned by the Alvarez family, the owners of an international company called Grupo Eulen, which provides services like security and cleaning to businesses in Spain, Portugal and Latin America. Since buying Vega Sicilia, the Alvarezes have diversified a bit, purchasing one Ribera del Duero winery called Alion, starting another in the nearby Toro region called Pintia, and opening a winery in Hungary called Oremus, specializing in sweet dessert wine.

The Vega Sicilia winery is as unusual as the wine itself. In an era of winery owners who strut like peacocks bragging about their wine, the head of Vega Sicilia, 55-year-old Pablo Alvarez, is shy and unassuming, speaking so softly that I strained to hear him even as I sat next to him at a restaurant outside of Valladolid, near the winery.

Rather than trying to duplicate the Cabernet- or Merlot-based wines of Bordeaux, as some Spanish wineries do, Vega Sicilia focuses its efforts on trying to take advantage of the unique properties of Tempranillo, the Spanish grape. The Unicos are at least 80% Tempranillo, which Vega Sicilia’s winemaker, Javier Ausas, calls “the best variety in the world,” even though it has practically no presence outside of Spain.

Tempranillo generally produces a softer, lusher style of wine than Cabernet, without the harsh tannins that are present in some young, high-end Cabernets. The soil and climate in Spain particularly suit the grape, says Mr. Ausas, and to produce a superb wine from Tempranillo requires a very low yield of grapes per unit of land.

Tempranillo also lends itself well to Vega Sicilia’s process of long aging in barrels. “Tempranillo has a phenomenal ability to age in cask, far beyond Cabernet,” says Terrance Leighton, a retired professor of microbiology at the University of California at Berkeley and the co-owner and winemaker of California’s Kalin Cellars. “Cabernet gets tired after three or four years in a barrel and starts to lose fruit and texture. Vega Sicilia can evolve with different sorts of complexities that wouldn’t be possible with Cabernet.”

Mr. Ausas notes that the winery is spending $25 million to build 60 new fermentation vats to replace the 21 now in use. The new vats, he says, will allow him to isolate different grapes grown in different compositions of soil, so that he can do a better job blending them together to make the wine. That big expenditure won’t produce any additional revenue, he points out, since production will remain the same.

At our lunch, Mr. Ausas opened bottles of the 2000 Unico, which will be released this year, and also the latest release of Especial, combining the 1991 vintage “for aroma,” 1994 “for power” and 1995 “for quality of finish.” Both were elegant—big and powerful, but with far more restraint than the “knock your socks off” expensive California Cabernets of small production that are so fashionable today.

But both versions indicated that there’s a lot more potential down the road as they open up with age. Since Vega Sicilia wines take so long to reach their peak, part of the challenge of drinking them is to project what they will taste like some day in the future. The oldest Vega Sicilia I’ve tasted was 1970—a friend shared one of his bottles with me last year—and it was so vigorous and youthful that if it had been poured blind I would have guessed it was a wine 10 years old.

When I asked Mr. Alvarez for a list of his favorite Unico vintages, he included 1989, because Vega Sicilia had made what he considered an excellent wine in a difficult year. A few nights later, I saw the 1989 on a wine list at a San Sebastian restaurant where I was eating dinner. At €256 ($350), it was barely more expensive than the current release of 1999 Unico at wine stores in Spain. But the 1989, I wrote in my notebook as I tasted it, was “nowhere, completely closed in, gives no hint of its potential.” In wine jargon, it could be called a victim of “sullen adolescence,” a transitory phase where the wine has lost its youthful exuberance but has not yet developed the enticing aromas and layers of taste of mature wine.

I should have known. After all, this particular Vega Sicilia Unico was only 20 years old.