EVEN IN A CITY where excavation sites are commonplace, and where watching hard hats go about their busy work in the depths is a favored pastime, the crater behind the Smithsonian Castle is exceptional.

Besides being huge -- 28,000 dump truck trips were required to haul away the 252,000 cubic yards of gravelly dirt that has been removed so far -- it is extraordinarily picturesque. The turreted castle and the polychrome Arts and Industries building hover on its edges, as if about to be swallowed up.

Both buildings would, in fact, fit comfortably in the giant void, though the castle's highest turrets would stick out. Not to worry. "We took great care to prevent ground movement that could have damaged the foundations of those buildings," reports Frank Gilmore of the Smithsonian's office of facilities services.

The method used was a slurry wall. For four months through last winter's bitter rains, workers fed the slurry, a thick liquid mixture of water and clay, into 60-foot-deep, 3-foot-wide trenches dug by a crane fitted with a narrow shovel called a clamshell. This guck, Gilmore explains, prevented the trenches from collapsing and stabilized the surface soil as well. After giant, prefabricated steel reinforcing cages were lowered, by sections, into the trenches, concrete was poured in by tubes from the bottom up, forcing the slurry out. Then came the bulldozers and the dump trucks. As the hole was being excavated, the walls were secured with tiebacks -- bundled steel cables drilled 60 feet into the surrounding earth and anchored in concrete.

But the most remarkable thing about this empty space is what will fill it up: not cars, not computers, but art. It almost takes the breath away to realize that sometime in 1987 we will be able to walk in a beautiful garden on this very spot, and, once inside architect Jean Paul Carlhian's elegant entrance pavilions (there will be two), step down into the big hole and feast our eyes upon the rare oriental artifacts in the Arthur M. Sackler Gallery and the treasures of the National Museum of African Art.