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1 .<-NARRATOR:-> Listen to part of a lecture in an archaeology class.
1 .<-FEMALE PROFESSOR:-> The Great Pyramid of Giza in Egypt might be the most famous building in the world.
2 .We know exactly when it was built-construction started in 2547 B.C.E.-about 4,500 years ago.
3 .We know who had it built,that was the Pharaoh Khufu,and we know who oversaw its construction: the Pharaoh's brother.
4 .We know so many things about it, but the funny thing is, we still don't know exactly how it was built.
1 .This picture'll give you an idea of the size of the pyramid …and the size of the blocks it's made out of.
1 .About two million stone blocks were used to build the Great Pyramid.And they're incredibly massive-the average weight is two and a half tons.
2 .The problem that has puzzled scholars for centuries is: How were these blocks lifted up the height of this massive structure and then fit into place? And without the benefit of modern technology?
1 .Of course there have been a lot of theories over the centuries. The oldest recorded one is by the Greek historian Herodotus.
2 .He visited Egypt around 450 B.C.E.,when the pyramid was already 2,000 years old.
3 .His theory was that cranes were used. Much like we use cranes today to construct tall buildings.
4 .And Herodotus may have seen Egyptians using cranes made of wood. But the problem with this theory has to do with simple mechanics.
5 .A crane needs a wide and sturdy base to stand on or it will fall over.
6 .Well, as you get toward the top of the pyramid, there's really no place for a crane to stand. The stone blocks are too narrow to provide a base.
1 .Well, so much for that theory.
2 .The next one has to do with the use of a ramp that would allow workers to drag a stone block up the side of the structure.
3 .Of course, the ramp can't be too steep. It has to have a long, gentle slope. And that's the problem.
4 .If you build a ramp with a slight slope up to the top of a pyramid that's over 130 meters high, it would have to be almost two kilometers long!
5 .Well, the pyramid is built on a flat area called the Giza Plateau.
6 .The plateau is simply not big enough to accommodate a two-kilometer long ramp.
1 .OK, so what now?
2 .Well, if you've ever driven on a mountain road, you know that it has a lot of twists and turns and bends in it because that's how engineers keep the road from having to be too steep.
3 .So why not wrap the ramp around the pyramid-building the ramp around it as you go?
4 .Sounds like a pretty good idea. Except it's got a serious problem.
5 .See, one of the most remarkable things about the Great Pyramid is how accurate the proportions are. The dimensions are almost perfect.
6 .To get that perfection, the engineers must have had to measure it repeatedly during construction.
7 .And the way you'd measure it is from the four corners of the base.
8 .Well, if you've got a ramp spiraling up from the base of the pyramid, those corners would be buried by that ramp during construction.
1 .Well, who says the ramp has to be on the outside of the pyramid? And now we get to the latest idea.
2 .If the ramp were on the inside of the pyramid,the corners at the base would be exposed, so the engineers could do their measurements while they were building.
3 .Well, an architect named Houdin has spent a few years working on making computer models of the building of the pyramid.
1 .And what Houdin believes is that an exterior straight ramp was used to construct the bottom third of the pyramid.
2 .This ramp would've been fairly short-it probably rose less than 50 meters.
3 .Then the rest of the pyramid was constructed using an internal ramp that spiraled around the inside of the pyramid.
4 .But how can we test this idea?
1 .Well, there are several ways to look inside the pyramid. One's called microgravimetry.
1 .Microgravimetry's a technique that's used to detect voids inside a structure.
2 .You can then take the data and generate an image that shows any empty spaces in the interior.
3 .Well, in 1986,French scientists completed a microgravimetric survey of the pyramid.
4 .And one of the images they produced showed an empty spiral-shaped space inside it.
5 .The shape of that space corresponds exactly to what Houdin thought the ramp would look like.
6 .I think Herodotus would be convinced. We might very well be at the end of centuries of guessing.