The riddle of which came first the Aepyornis or the egg - remains unsolved, but science now knows what you get when an egg sits around for 2,000 years.

Dust.

Radiologists at University Hospital placed the calcified egg of an extinct Aepyornis bird in a CAT scanner Tuesday to see whether the football-sized shell - the largest known egg of any organism in the world - still held an intact embryo.CAT scanning, or computerized axial tomography, uses a computer to generate thin cross-sectional images of tumors and other tissues normally blocked from view by bone and other dense materials in traditional X-rays.

Researchers carefully moved the 20-pound egg beneath the machine's eye. Using the same techniques employed to diagnose illnesses in humans, technicians bombarded the egg with weak X-rays. Within minutes, the electromagnetic radiation had penetrated the egg's 4-millimeter-thick shell and searched deep into its core.

But instead of a preserved chick, all that appeared on the scanner screen was an egg outline filled with air and a thin pile of powdered bird and egg white at the bottom.

"We're pretty far into the densest part of that mass, and that's what's in there," said Ed Hendrick, director of radiological sciences at the University of Colorado Health Sciences Center.

"It's not like a well-defined embryo. It looks like fragments of bone and calcified stuff."

The egg was laid 2,000 years ago by an Aepyornis, a 10-foot-tall bird that once roamed the island of Madagascar.

The Denver Museum of Natural History purchased it in 1930. It is one of only two dozen of its kind remaining in the world. The egg will be part of a special museum exhibition that opens March 18.