NARRATOR：

Listen to part of a lecture in an archaeology class.

FEMALE PROFESSOR：

The Great Pyramid of Giza in Egypt might be the most famous building in the world.

We know exactly when it was built-construction started in 2547 B.C.E.-about 4,500 years ago.

We know who had it built,that was the Pharaoh Khufu,and we know who oversaw its construction: the Pharaoh's brother.

We know so many things about it, but the funny thing is, we still don't know exactly how it was built.

This picture'll give you an idea of the size of the pyramid …and the size of the blocks it's made out of.

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.

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?

Of course there have been a lot of theories over the centuries. The oldest recorded one is by the Greek historian Herodotus.

He visited Egypt around 450 B.C.E.,when the pyramid was already 2,000 years old.

His theory was that cranes were used. Much like we use cranes today to construct tall buildings.

And Herodotus may have seen Egyptians using cranes made of wood. But the problem with this theory has to do with simple mechanics.

A crane needs a wide and sturdy base to stand on or it will fall over.

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.

Well, so much for that theory.

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.

Of course, the ramp can't be too steep. It has to have a long, gentle slope. And that's the problem.

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!

Well, the pyramid is built on a flat area called the Giza Plateau.

The plateau is simply not big enough to accommodate a two-kilometer long ramp.

OK, so what now?

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.

So why not wrap the ramp around the pyramid-building the ramp around it as you go?

Sounds like a pretty good idea. Except it's got a serious problem.

See, one of the most remarkable things about the Great Pyramid is how accurate the proportions are. The dimensions are almost perfect.

To get that perfection, the engineers must have had to measure it repeatedly during construction.

And the way you'd measure it is from the four corners of the base.

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.

Well, who says the ramp has to be on the outside of the pyramid? And now we get to the latest idea.

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.

Well, an architect named Houdin has spent a few years working on making computer models of the building of the pyramid.

And what Houdin believes is that an exterior straight ramp was used to construct the bottom third of the pyramid.

This ramp would've been fairly short-it probably rose less than 50 meters.

Then the rest of the pyramid was constructed using an internal ramp that spiraled around the inside of the pyramid.

But how can we test this idea?

Well, there are several ways to look inside the pyramid. One's called microgravimetry.

Microgravimetry's a technique that's used to detect voids inside a structure.

You can then take the data and generate an image that shows any empty spaces in the interior.

Well, in 1986,French scientists completed a microgravimetric survey of the pyramid.

And one of the images they produced showed an empty spiral-shaped space inside it.

The shape of that space corresponds exactly to what Houdin thought the ramp would look like.

I think Herodotus would be convinced. We might very well be at the end of centuries of guessing.