Among the seven wonders of the ancient world, the Pyramids at Giza, Egypt, are the only ones still standing.
For 1,000 years, they were the tallest buildings in the world. Built with prehistoric technology, they were an architectural riddle, as elusive as the Sphinx.
No one really knows how they were built. The first historical record, by the Greek writer Heroditus, came 2,000 years later. In the absence of hard facts, there have been many theories.
Hollywood has mostly taken its cues from the Greek account: 100,000 slaves laboring for 20 years.
Others believe a giant ramp snaked around the pyramid.
One American recently advanced the theory that kites could have lifted the 2-ton stones.
Now a French architect says that after studying the pyramids for eight years, he has solved the mystery with the help of 3-D computer animation.
Jean Pierre Houdin believes the pyramids were built from the inside out, through an internal spiral ramp.
"This is completely new. Everyone [before] me thought that the pyramid was built from the outside, only the outside," Houdin said.
Scholars of ancient Egypt are intrigued.
"It's a plausible theory. Egyptologists agree it ought to be tested, and it's nondestructive, so I wouldn't be surprised that within a year, we have the final answer," said Bob Brier, an Egyptologist at Long Island University, C.W. Post.
If that is the case, we'll know the truth about the pyramids in the blink of an eye, considering this mystery has endured for nearly 5,000 years.