Greek historian Herodotus, writing around 450 B.C., theorized the use of small, wooden, cranes or levers to lift the blocks.
But, Brier said, "you'd have to have thousands, and they didn't have enough wood in all of Egypt for that," Brier said.
For Houdin, the Paris architect, the puzzle of the pyramid is a family affair. His father, a civil engineer, came up with the idea of an internal construction ramp a decade ago.
Houdin was soon hooked, as suggested by his recent book, co-written by Brier—The Secret of the Great Pyramid: How One Man's Obsession Led to the Solution of Ancient Egypt's Greatest Mystery.
Houdin eventually left his architecture firm to pursue the inside-out theory full-time.
For what they thought would be a matter of weeks, he and his wife moved into a 236-square-foot (22-square-meter) studio apartment. They ended up staying for four years, as Houdin toiled away at his self-financed project.
Outside Ramp, Then Internal Tunnel
Houdin's theory suggests the Great Pyramid was built in two stages.
First, blocks were hauled up a straight external ramp to build the pyramid's bottom third, which contains most of the monument's mass, Houdin believes.
Houdin says the limestone blocks used in the outside ramp were recycled for the pyramid's upper levels, which might explain why no trace of an original ramp has been found.
Egyptian-archaeology specialist Josef Wegner sees merit in the recycling idea.
"The notion of using the already quarried smaller blocks to build the lower ramp and then dismantling that for use in upper sections would be a very logical approach to speed up the overall construction process," said Wegner of University of Pennsylvania Museum of Archaeology and Anthropology.
After the foundation had been finished, workers began building an inclined, internal, corkscrew tunnel, which would continue its path up and around as the pyramid rose, Houdin said.
Because the tunnel is inside the pyramid, Brier said, "when they finished getting blocks all the way up to the top this ramp disappeared [from view]."
New Clue: The Hidden Room
New evidence uncovered about two-thirds of the way up the Great Pyramid supports the inside-out theory, said Houdin, the architect.
At about the 300-foot (90-meter) mark on the northeastern edge lies an open notch.
On a recent expedition with a National Geographic film crew, Brier—aided by a videographer with mountain-climbing experience—scaled perilous crumbling rocks to reach the notch. (The National Geographic Society owns National Geographic News.)
Ducking inside the notch, Brier entered a small L-shaped room.
He wasn't the first to visit the space, but until now Egyptologists had taken little notice of it.
Houdin, the architect, said the feature figures perfectly with his theory.
Open Corners for Turning Blocks?
For the interior tunnel to work, it would have required open areas at the Great Pyramid's four corners, Houdin says. Otherwise the blocks wouldn't have been able to clear the 90-degree turns.
Like railroad roundhouses, these open corners would have given workers room to pivot the blocks—perhaps using wooden cranes—so the stones could be pushed into the next tunnel.
The notch and room are remnants of one such opening, Houdin claims. They are located at one of the spots where Houdin's 3-D computer models suggest they should be.
Inside the corner space, which was apparently walled in as the pyramid was completed, there should be two tunnel entrances at right angles to one another—each leading to a section of the internal ramp, Houdin believes.
Perhaps all that stands between him and the solution to the mystery are massive blocks that thousands of years ago sealed the tunnel, Houdin said.
If this previously known space truly is the missing link in the puzzle of the Great Pyramid's construction, the question remains why no one would have surmised this by now.
Brier said, "If you weren't thinking about internal ramps and notches and you climbed right by this thing, it wouldn't mean anything to you."
The Other Key Clue
Prior to the room brainstorm, Houdin's most important piece of evidence was the product of good luck.
In 1986 a French team in an ultimately fruitless search for hidden chambers in the Great Pyramid had done a survey of the monument's density using a technique called microgravimetry, which measures the strength of local gravitational fields.
Nearly 15 years later, Houdin was presenting his ramp theory at a conference and was approached by a member of the 1986 team.
The man showed Houdin an image from their survey that they'd dismissed as unexplainable.
But to Houdin, and later Brier, the explanation was clear.
The image shows what looks like a spiraling feature inside the structure's outer walls.
"If I hadn't seen that diagram, I'd probably be thinking this is just another theory," Brier said.
Next Step: Confirmation
The 1986 image, the notch room, and other evidence may make Houdin's theory plausible, but the case is far from closed.
"As with all archaeological theories, the proof is in the pudding, and many logical and compelling theories have fallen by the wayside under the weight of hard evidence," said the University of Pennsylvania's Wegner.
But "verification of the proposed internal spiral ramp would be a remarkable and groundbreaking discovery," Wegner added.
Houdin believes that verification might soon be possible.
He suggests that an infrared camera—positioned about 150 feet (46 meters) from the pyramid—could potentially record subtle differences in interior materials and temperatures. Those variations could reveal clear-cut "phantoms" of the internal ramp.
"What we need is the authorization, by the Egyptian authorities, to stay around for 18 hours, close to the pyramid, with a cooled infrared camera based on an SUV and to take images of three [pyramid] faces every hour during this period," Houdin said.
"A green light from Cairo and the Great Pyramid mystery is over."
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