A woman joked that the three-ton slab of marble, sunk in the earth like a crashed UFO, was part of a new National Gallery of Art classical antiquities exhibition.

It wasn't part of an exhibit, though. It was part of the gallery.

The 6-by-9-foot slab fell unnoticed from the top of the National Gallery at 6th Street and Constitution Avenue sometime Monday, officials say.

Earlier that day gallery employes noticed that part of the building's right cornice was missing. However, the triangular slab, a weathered concrete-gray color, wasn't discovered until security guard Ronald Estes found it while making after-hours rounds at 10:50 Monday night.

The marble fell behind a wall at the side of the gallery entrance, just beyond some shrubs in a clear patch of grass where visitors are not allowed. There were no injuries, gallery officials said. Nor, apparently, was there any warning.

"Nobody ever saw anything," NGA press aide Katherine Warwick said yesterday.

Officials called the incident "an utter irregularity, an inherent fault in the marble" that has nothing to do with "structure, neglect or vandalism."

"That's the only explanation we have to date," Warwick said. "By no means is the building falling apart."

Bill Swenson, a stone contractor with Roubin and Janeiro, said the marble could have cracked as a result of years of weathering. Chances of it happening again are low. "It's most unusual," he said.

The gallery, designed by Jefferson Memorial architect John Russell Pope, was completed in 1941. The same type of pink Tennessee marble used in the building was also selected for the gallery's new East Wing.

The gallery's Constitution Avenue entrance and driveway were closed two hours yesterday morning while Smithsonian maintenance crews and local stone contractors checked the exterior and deemed the building safe. Structural engineers will examine the entire outside today, Warwick said.

The gallery does not plan to take any other precautions.

"Its such an unusual occurrence we don't believe it can happen again. We're just very, very grateful no one was hurt," Warwick said.

As for the slab, gallery officials are considering placing it back in position. For now, it is hidden behind the wall from public view where it may remain for a while.

"It's still back there," a guard said yesterday. "Ain't nobody gonna lift it."