Construction won't begin until next year, but the buzz surrounding Loudoun County's most anticipated new building in decades got a lot louder this week when architects and planners formally laid out their vision for a cascading, glass-and-earth home for the Howard Hughes Medical Institute's $500 million research campus.

The 750,000-square-foot biotech facility, which will be imbedded -- hidden, even -- in hills between Route 7 and the Potomac River south of Leesburg, was designed by New York-based Rafael Vin~oly Architects, one of the firms working on designs for rebuilding the World Trade Center.

For 40 years, Dulles International Airport's terminal, with its swooshing ramp soaring over paved pasturelands, has been Loudoun's most prominent architectural landmark.

Backers of the Hughes project, which will be nestled near a 1928 French manor house on a 281-acre site called Janelia Farm, said they have similarly lofty aspirations for the low-slung "landscape building" that will house a network of laboratories, meeting spaces and overnight accommodations for physicists, chemists, and computer scientists hunting for biomedical breakthroughs.

"It's going to be the biggest invisible building you've ever seen," said Thomas R. Cech, president of the institute, which is backed by a $11 billion endowment from Howard Hughes, the late aeronautics magnate and film producer.

Cech, keenly aware of the tumult in Loudoun over development in recent years, emphasized that he and his colleagues hope the project serves as a model for the environmentally sensitive design and construction practices that county supervisors say they hope will be a hallmark of their "smart growth" agenda.

"We're hoping we will be seen as not just someone that's in line with this, but as a leader," Cech said.

Jay Douglas Bargmann, a senior vice president at Vin~oly, said the facility will include offices for principal investigators, an auditorium and in-house hotel, 330 underground parking spaces and research rooms with high ceilings and industrial floors that can withstand the weight of trucks delivering the medical instruments of the future.

Many of the structures, which will be terraced into the landscape and surround linked ponds, will be covered with two feet of dirt and planted greenery, helping insulate the facility and anchoring it in the surroundings, he said. Sections overlooking the pond will be mostly glass, creating an airy feeling.

"The building integrates itself with nature," Bargmann said. "Everything is open and meant to promote interaction and a sense of community."

County officials gushed over the plans when they were presented Monday, and over the prospect of a leading medical research group basing a major operation in Loudoun, which is looking for new economic engines to replace ailing telecommunications firms.

Board Chairman Scott K. York (R-At Large), a home-improvement contractor who has benefited politically from anti-development sentiment, praised the Hughes development as a "wonderful project."

Virginia's Commerce and Trade Secretary Michael J. Schewel was even more effusive.

"It's good for Loudoun County. It's good for the world," and Virginia's biotech sector, Schewel said. "I think it's an all-around home run."

Officials in Montgomery County, where the institute's Chevy Chase headquarters are located, are disappointed Hughes chose Loudoun for its expansion, institute representatives noted. But the large, scenic plot within an hour of its headquarters and near the Dulles Airport made the Loudoun site too attractive to pass up, Hughes officials said.