It might be the set for the next Steven Spielberg movie or perhaps one of those wild "environmental" works of art. Rising from the earth on an ordinary Fairfax County road is a collection of surrealistic bunkers. But beneath the bunkers and the grass, about 100 employees of the Fairfax County Redevelopment and Housing Authority toil not on a movie set but in the area's first underground office.

The 18,000-square-foot building is tucked away in a residential neighborhood near the George Mason University campus. The south face of the building is exposed to the roadway, but, with the exception of a small entryway on the north side, the rest of the building is submerged -- the roof is grasscovered and a regional bike path runs across it.

Architect Walter F. Roberts planned the Housing Authority building to be both unobtrusive and a good example for energy conservation. Roberts maintains that the energy costs of his underground building are about one-third those of a conventional building of comparable size.

The $1,186,000 structure is a remarkably light and airy place to work, thanks to the bunkers on top of it. The 16 little houses, centered over bays, are called light monitors by the architect. The monitors flood the offices below with indirect, natural light -- and it's all done with mirrors. Each minitor has two mirrors -- one to catch the high summer sun and one for winter sun -- that reflect the outside light and bounce it into the space below.

But the monitors, each about 14 feet above the office ceilings, serve another purpose equally important to catching sunlight: They store warm air. In summer, a special thermostat releases that air from vents at the top of the monitors to keep the building cool. But in winter, the warm air collected all day spills inside and stays, like an invisible blanket, just under the ceiling to keep the building warn. The roof is covered with four inches of insulation, a blanket of rubber one-sixteenth of an inch deep and 18 inches of earth. It leaks "only in the corners of the monitors," says Roberts, where the flashing was incorrectly sealed. Repairs are under way.

The building's heating and cooling system is operated by a heat pump that in winter functions like a reverse air conditioner. It takes outside air, heats it in a coil and distributes it throughout the building.

In terms of energy conservation, the building is a tour-deforce of passive solar techniques. Passive solar design means that the structure is constructed to take advantage of the sun's warmth without benefit of a lot of fancy gizmos, such as solar collector panels. Roberts estimates that the building cost $90,000 more to build than a comparable traditional structure would have cost, but he believes the energy savings will pay for the extra cost within the first four or five years.

The combination of a wall of south-facing windows (with a built-in redwood shield against direct glare) and the 16 light monitors allows natural light to pour into the building. But what about cloudy days? Lighting consultant Peter Barna came up with a system of photoelectric controls that help to keep the fluorescent lights evenly balanced at a normal daylight level so that there are no dark spots in the office. Barna also placed a set of fluorescent tubes which diffusing fins around the interior of every light minitor so that these large square holes in the ceilings are always illuminated, even on dark winter days and in the evenings. The natural and artificial light work together: If the day grows dark, the artificial light inside will compensate.

In late May, Roberts began a study of worker response to the daylighting system and found that, overall, employees liked the system and weren't more aware of or distracted by the outside than they had been in a more traditional office.

Lighting is not the only reason one doesn't feel buried in the building. Clearly, 14-foot ceilings throughout give an expansive sense of space. And most of the employees work in an open-plan office, allowing the light from the front of the structure to penetrate the full depth of the building. Even the few enclosed offices have clerestory windows above the standard 8-foot high mark to allow light to filter into the office.

The atmosphere inside the offices is subdued. After many visits by agency director Walter Webdale to new public and private offices, he and architect Roberts came up with a neutral color scheme for the interior. Muted beige is broken only by subtly decorated blue columns with a touch of redwood molding. Both Roberts and Webdale concluded that the best way to keep the place open and uncluttered was to adopt strict rules about what could be displayed on walls. At the same time, they took pains to design amenities that they associated with private industry -- simple things like mini-kitchens for coffee breaks, an attractive lunchroom and picnic tables for the rooftop.

Though there were a lot of complaints about the openness of the building plan when it was ready for employees last November, people seem to have adapted well to working underground. High ceilings, pleasant amenities, natural light and a bank of windows have helped to keep employees productive and happy, as well as truly down to earth.