Easter egg hunts may have come and gone, but bidding will open soon on one of the world's largest eggs: On April 24, Christie's auction house will give buyers a crack at nabbing an elephant bird egg that's a foot (30.5 centimeters) long and nearly nine inches (22.9 centimeters) in diameter.

The elephant bird egg will be featured during Christie's Travel, Science and Natural History sale in London. The auction house expects the egg to go for more than $45,000.

Extinct since the 17th century, elephant birds were found only on the island of Madagascar. Researchers believe the birds went extinct because of human activity—primarily habitat loss and hunting.

The flightless, ostrich-like birds could grow to be 11 feet (3.35 meters) tall. And their eggs were huge—up to 100 times larger than the average chicken's. Intact eggs today are considered extremely rare and can demand steep prices.

"Any whole eggs found today in Madagascar are legally the property of the Malagasy government," said archaeologist Mike Parker Pearson, a professor at the Institute of Archaeology, University College London. "So no more complete eggs will ever come on the open market, and that must make these very few eggs already in museum and private collections very valuable."

So I was intrigued when I learned, in the course of researching this story, that National Geographic owns one of these rare eggs. I decided to stage my own egg hunt and set off to find it.

The egg had originally been given to Luis Marden, a legendary photographer for National Geographic magazine, in 1967 when he was on assignment in Madagascar—just one of many assignments around the world over his six-decade career with the magazine.

When he was just 19 Marden wrote one of the first books on 35mm color photography, Color Photography with the Miniature Camera. He would go on to become an early pioneer in underwater color photography; many of his underwater camera techniques are still used today.

After email inquiries, phone calls, and snooping around random rooms in the headquarters building in Washington, D.C., I got word of the possible location of the egg. A tip led me to a staff member at the National Geographic Museum.

"I'm looking for an elephant bird egg that was given to Luis Marden in 1967," I queried.

"Ah! The Aepyornis egg that Luis Marden obtained while on assignment in Madagascar," said the representative, who wishes to remain nameless.

Once out for display at the National Geographic Society's Washington headquarters, I learned, the egg now rests in a locked case in a climate-controlled room underground.