Energy economy is not the only reason to bury a building.
Minneapolis architect David Bennett discovered so many other advantages to earth shelters when designing the city's new Walker Library that energy conservation became just one more benefit.
For one thing, going underground meant the city could use a smaller site, resulting in a saving of nearly 9 percent of the construction cost.
The ground-level rooftop parking lot makes Walker more accessible to the handicapped and solves the usual parking problem associated with small lots. In fact, there was enough room left over for a community plaza.
The library was built around, or rather below, the plaza, originally envisioned as an open urban area and capstone for a mall extending to nearby Lake Calhoun and Lake of the Isles. Designed specifically for the local annual art fair, Mr. Bennett admits the plaza may have a significance ''out of proportion to its short lifespan.''
Besides the plaza, Walker's sunken patio lets light spill down into the subterranean stacks. Strategically located mirrors and skylights carry light into the interior, reducing costs of fixture lighting.
Perhaps the greatest benefit to being down under, from the library's point of view, is soundproofing. Sinking the reading area drowns out distracting street-level noise.
And finally, the advantage of energy conservation can't be ignored.
A small residential heater keeps the reading area, meeting rooms, and staff facilities a comfortable temperature during the harsh Minnesota winters right down to the -2 degrees F. level. The same principle of earth-contact temperature stability keeps the building cool in summer.
Walker Library showcases Bennett's site-specific philosophy toward architectural problems. The Minneapolis library board and nine community groups took six years to settle on the small site at the corner of Hennepin and Lake, bridging the neighborhood's commercial center and the city's system of lakes and parks.
When the park and recreation board refused to make additional land available, Mr. Bennett had the choice of fitting at least 18,000 square feet of building on just 20,000 square feet of ground or facing more years of grueling site debate.
Cost savings, space efficiency, and noise reduction were reasons in and of themselves to put Walker Library underground.
But Mr. Bennett doesn't believe earth shelter is the only way to build.
''Architects, especially American architects, have a propensity to seize upon an architectural solution and wave it around as a panacea for everything,'' he says. ''Earth shelter is not a solution to every site and every condition. This kind of architecture is appropriate for certain sites and special conditions.
''In the case of Walker Library we had to put a large building on a small lot , so we decided to go underground, putting the building under its own parking lot.''