Underground building sounds straightforward enough. Obviously, it means a building that's underground. Yet, when it comes to creating a list of public and commercial underground buildings in the United States, categorization problems crop up almost immediately. How underground must a building be, to be an underground building?

One aspect of the problem is illustrated by the Silvestre S. Herrera Elementary School in Phoenix, Arizona. Like many earth-sheltered buildings, this one-story structure rises approximately one-half story above ground level. Most of the wall surfaces are bermed (i.e., an embankment of earth is sloped up to cover all or part of each wall). The roof is not covered with earth, but with concrete. As students run up the gently sloped, landscaped ground from a play yard to the rooftop basketball courts, they are oblivious to the school building beneath their feet. On the other hand, people approaching the front door of the school see a typical, although low-profile, facade and enter it in a normal way after walking down only a few steps from the parking lot. This building seems more deserving of being labeled underground than one that is built 1 foot below grade and has soil bermed only 3 feet up the exterior walls. But where is the dividing line in terms of a definition of underground building?

The confusion might not even be cleared up by restricting the discussion to buildings that are literally "under ground" by virtue of their roofs being covered with soil. Some earth-topped buildings are merely that: aboveground buildings crowned with "living roofs" of vegetation. A truck factory currently under construction at the Ford Rouge Center in Dearborn, Michigan, is one example. The 600,000-square-foot building, which rises from ground level, will be topped with a field of ivy, and vines will be encouraged to cover its exterior walls. That will not transform the structure into an underground building. A less-clear example is presented by a student housing complex at St. John's University in Collegeville, Minnesota. Each of its five buildings consists of four 2-story apartments. At the front of the building, the upper story is at ground level, and the ground slopes down to daylight the lower level. The grass-covered roof slopes down toward the back of the building, where it seamlessly joins the surrounding lawn. An impression of whether these are underground buildings may depend on whether the viewer looks at them from the front or the rear.

Another area of confusion is created by the use of basement space under aboveground buildings. Skyscrapers, for example, often have two to six basement levels housing retail stores as well as subway stations, building support spaces, and parking garages. Should a shopping center under a tall building be considered an underground mall? Does it depend on whether the mall extends beyond the perimeter of the building, occupying space under an outdoor plaza or an adjacent street? If you conclude that below-grade levels directly underneath above-grade buildings should not qualify as underground buildings, consider a less clear-cut example, Building K of the CamelSquare Executive Suites complex in Scottsdale, Arizona. What appears on the surface to be a one-story office building is actually a four-story structure arranged around a pair of roofless atriums. An open-air corridor in a lower level of the building, shielded from traffic noise and insulated from scorching summer heat, is a markedly different setting than an exposed hallway on the ground floor of a four-story above-grade structure.

Finally, and perhaps easiest to resolve, is the case of a building located at or above the surface level of its entrance, but enclosed in the earth. One of the best illustrations of this phenomenon is the Brunson Instrument Company factory and headquarters in Kansas City, Missouri. Hollowed out of a limestone bluff, the 125,000-square-foot facility is a half a dozen steps higher than its adjacent parking lot. On the other hand, it sits under 75 feet of solid rock, and its walls are enclosed by at least 15 feet of stone. In-ground? Definitely. Underground? Probably.

Because of the ambiguity of the term underground building, the database posted on this site contains some debatable listings. The problem, however, is more serious than deciding which buildings belong on a list. For the purposes of clarifying their building and zoning codes, for instance, various jurisdictions have chosen their own definitions. The Land Use Code for the city of Bellevue, Washington, includes buildings "that rise up to 30 inches above existing or finished grade, whichever is lower" in this category, for instance. The zoning ordinance of Port Washington, Wisconsin, is more restrictive, granting automatic exemptions for property line setbacks, building size, and open space requirements only for buildings "not extending more than 6 inches above the surface of the ground." Cities in the Kansas City area, where installations like the Brunson Instrument Company's are more common, define underground space as the "cavern resulting from the extraction of subsurface-located material in such a manner that the surface area of the property is not disturbed except in the vicinity of the entrances and easements servicing the development."

At least two agencies of the federal government have formulated definitions that would apply nationwide for specific purposes. "Financing Earth-Sheltered Housing," a report prepared for the US Department of Energy, says, "Broadly defined, an earth sheltered house has at least 50% of its entire exterior envelope, often including the roof, in direct contact with earth." For its National Flood Insurance Program, the Federal Emergency Management Agency (FEMA) considers an underground building to be "a building for which 50 percent or more of the actual cash value, including machinery and equipment that are part of the building is below ground."

Some words or concepts do not lend themselves to clear, concise definitions. In concurring with a 1964 US Supreme Court decision concerning obscenity, Justice Potter Stewart addressed the problem of "trying to define what may be indefinable." Rather than embarking on a potentially futile effort to create such a definition, he concluded that "I know it when I see it." In the realm of the built environment, architects and others have similarly struggled to specify what a skyscraper is. When a reporter for the Christian Science Monitor (July 1998) asked for such a definition, architect T.J. Gottesdiener replied, "I don't think it is how many floors you have. I think it is attitude." Gottesdiener is a partner in Skidmore, Owings & Merrill, a firm that has designed numerous very tall buildings including the Sears Tower. He extended his answer this way: "What is a skyscraper? It is anything that makes you stop, stand, crane your neck back, and look up."

I wonder what the analogous description would be for an underground building.

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