Giant blocks, some weighing a hundred tons, sit next to each other so precisely that not even a razor blade can fit between them.

Without iron tools, draft animals, or the wheel,

how did the Inca builders move and set such large blocks?

To answer this question,

NOVA invited several experts with widely different backgrounds to come to Peru.

We're pretty good at finding the evidence today,

Professor of architecture Jean-Pierre Protzen studies the Incas' use of stone.

He has written a book about Inca architecture and has some definite ideas about their construction methods.

Ed Franquemont is both an anthropologist and a building contractor

who lived in a Peruvian village for several years.

His particular interest is how the Inca builders organized their labor force.

Vince Lee is an architect and explorer who has travelled extensively in the Andes looking for lost Inca sites.

He has a theory about how the Inca stonemasons made such precise joints with such giant stones.

A good place to start looking for clues is the citadel overlooking the town of Ollantaytambo.

About five hundred years ago, a sun temple was under construction inside the fortress.

ED FRANQUEMONT: So with all these blocks of stones here, this is clearly a construction site that was abandoned in progress.

ED FRANQUEMONT: The question is, where do these stones come from, and how did they get here?

JEAN-PIERRE PROTZEN: Well, they came from the quarries

on the other side of the river at the base of this mountain here.

The team decides to follow the route to the quarry taken by the ancient stone haulers.

The hike will take them down a sloping ramp to the valley floor.

Along the way, they find massive blocks abandoned by the Inca workers.

The villagers called these rocks "piedras cansadas"-weary stones.

One legend tells of stones that grew tired, wept blood and refused to move.

JEAN-PIERRE PROTZEN: And so is there

so this is where the quarry is, the lower quarry at the bottom of this rockfall.

You can see some ramps. The other quarry is way up there at the foot of the cliffs.

Jean-Pierre-JP to his friends-leads the team along the remains of a roadway that leads to the quarry

actually a rockfall created by rocks eroding from the cliffs above.

Here, they find a 70-ton stone that Inca quarry workers had turned into a rectangular block.

JP believes that all the boulders were first squared off in the quarry.

But how did the Incas transport these heavy blocks down the mountain

and up to the sun temple on the other side of the valley?

Spanish chronicles tell us that the Incas did not possess the wheel or strong draft animals like oxen.

David Canal, a community leader and Inca descendent, believes they hauled the blocks by hand.

He has organized a team of pullers to transport a one-ton rock along

the same route taken by the Incas between the quarry and the citadel.

For most of its length, the ramp has a gentle slope.

But halfway down the mountain, the incline suddenly turns into an almost vertical 800-foot chute to the valley below.

With a block more than ten times the size of this one,

it must have been extremely difficult for the Ancient stone haulers to negotiate this chute.

Unlike the Inca blocks observed on the transport route, this boulder has not been squared off,

and it tumbles out of control.

ED FRANQUEMONT: Probably not the way the Incas wanted to see it happen.

VINCENT LEE: No. No.

VINCENT LEE: And you know, once it-if it turned this way, it was kind of cylindrical.

It was kind of easy for it to get rolling, where a big square block might not have-

JEAN-PIERRE PROTZEN: That's true. -might not have done that.

Having gotten the boulder down in pieces by a distinctly non-Inca method,

everyone hopes to do better with the next challenge:

getting a block across the Urubamba River.

At this time of year, the water level is at its lowest, and the river looks quite placid.

But after the rainy season, it becomes a torrent impossible to ford.

David believes the Inca hauling teams would have chosen to cross at the shallowest stretch.

But even here, there is a stiff current, and many of his men can't swim.

To appease the spirits of the river,

David has arranged for an offering of cane alcohol.

The wet stones are slippery for the men,

but this turns out to be an advantage when it comes time to pull the rock.

ED FRANQUEMONT: It was perfect! It was easier than moving it no the ground, went pretty quick.

Just exactly like I thought it would.

The task of getting rock across the Urubamba turned out to be much easier than everyone had imagined.

But crossing the fields on the valley bottom is much more of a problem,

because the stone acts like a plow digging into the soft ground.

There was probably once a road crossing the valley, but it has been destroyed by centuries of farming.

Permission has been obtained to excavate one of the blocks abandoned in Inca times,

to see if there is any evidence of a roadbed underneath.

VINCENT LEE: What turned up underneath was a layer of small stones

on top of what appears to be a prepared gravel road base.

So the resulting surface that the stone appears to have actually been bearing on is just these stones

about the size of a softball-not necessarily round,

but you know-and that's not unlike the surface we find on the ramps today, still.

Now that they have found the kind of road used by the Inca stone haulers,

the team wants to see how difficult it would be to drag a much heavier block on a similar surface.

In the plaza below the citadel, they find a genuine fifteen-ton Inca block,

and the sloping cobbled surface is a good approximation of the eight-degree ramp that leads up to the sun temple.

To pull the block, David has assembled a team of two hundred and fifty men,

women and children from Ollantaytambo and neighboring villages.

There is a festive atmosphere.

Everyone has turned out to see the great block being dragged through town.

Unfortunately, the stone refuses to budge.

But after another offering of cane alcohol, and some levering, the stone finally comes unstuck.

The ease with which the block travels on the cobbled surface proves

that it could have been dragged up the slope to the sun temple.

DAVID CANAL: (English translation) I had no doubt that we could do it.

Our ancestors did it, so I knew we could do it, too. Human labor can accomplish anything.

The determination displayed by David's people makes the speed and

scale of the Incas' empire building achievements much more understandable.

According to legend, around 1450 A.D., a leader called Pachacuti, whose name means "Earth-Shaker",

began an aggressive military campaign that transformed the Incas from a small Cuzco valley community

into a juggernaut that swallowed up all its Andean neighbors.

In return for the benefits of a stable state, conquered peoples paid tax to their Inca masters in the form of labor.

This huge workforce enabled Pachacuti and his successors to build the infrastructure

that could support their rapidly expanding territorial gains.

In the Urubamba valley, wide, rambling sections of the river were placed in canals to create cultivatable land.

Terraces watered by elaborate irrigation schemes climbed the mountainsides,

further increasing food production.

On the peaks above the Urubamba River,

the Inca lords built a chain of remarkable citadels in the sky.

The most magnificent and mysterious of all: Machu Picchu.

ED FRANQUEMONT: It's difficult for us to grasp the scale of the Incas' imagination and ambition in producing places like this.

As archaeologists, we like to work with potsherds or tools or walls or buildings

-things that are people scale.

But the Inca's vision was much bigger than that.

The real Inca media was the entire, immense Andean landscape around him.

He spent extra time to find very special places within the Andean landscape,

spent time studying them to understand their true nature,

embellished them with stone,

ran sparkling and rushing water through prepared water courses,

and in the end, produced works of singular beauty that represent a harmony

with nature that few other civilizations have achieved.

So remote was its location, Machu Picchu's existence remained a secret

from the time of the Incas until the early part of this century.

But thirty miles up river, the town of Ollantaytambo has been lived in continuously since the time of the Inca.

Its buildings are well-preserved.

But the very finest Inca stonework is found in the citadel above the residential quarters.

Replicating joints like this is the challenge JP Protzen and Vince Lee have set themselves.

VINCENT LEE: You know, JP, this part of Ollantaytambo has always been one of my favorites.

I mean, this is Inca stonemasonry as good as it gets. Don't you agree?

You're right. You bet.

It's not just the craftsmanship. It's just the playfulness of the joining

-and the problem that they elected to solve is just so complicated. It's wonderful.

JEAN-PIERRE PROTZEN: Yes. Yeah. I mean, you really see that here, they perfected their skills.

Yeah. And you know, the other thing it seems to me, that where other cultures used stone

as a material for sculptural decor of one kind or another, these guys just use the stone itself.

They're just telling you that stone is itself a beautiful material.

You don't have to carve anything into it, really.

No. No. This is sculpture, too.

You know, people often say, oh, you can't get a knife blade in the joints of Inca

-you can't get anything in this JPP: Not even a razor blade.

No! No, it's an absolute perfect joint.

I mean, the craftsmanship is mind-boggling, especially if you try to do it-

if you try to duplicate it yourself!

JP has duplicated Inca stonework using Inca tools.

In an ancient quarry, he discovered some rounded stones that probably came from the river.

Using these as hammerstones, he found them as effective as the modern steel chisels used by stonemasons today.

To create a bevelled edge, JP used a smaller hammerstone.

The resulting tool marks are identical to those found on Inca masonry, rough in the center and smooth at the edges.

But how did the Inca masons go about setting the stones?

A half-finished citadel wall provides an important clue.

To achieve the perfect Inca joint, an imprint is marked on the block below.

The area that will seat the new block is then hammered out.

Repeated fittings fine-tune the joint.

Spots where stone dust is compressed indicate raised areas that need more hammering.

Using ever-smaller hammerstones to avoid damaging the edges

JP finished the joint within a few hours.

JEAN-PIERRE PROTZEN: It shows that with the sort of simple tools that I found in this quarry,

it is absolutely possible to achieve the kind of perfection of stonework that we

observe throughout Cuzco and the Inca empire.

JP's method works well with small stones that can be easily maneuvered.

But as the stones get bigger, handling them becomes increasingly difficult.

Here at the Inca fortress of Sacsahuaman, the trial and error method of setting giant multi-ton blocks

seems a daunting prospect.

But despite their size, the blocks in the retaining wall all have the famous Inca fit, mortarless and snug.

The answer may be a simple builder's tool called a scribe,

a tool that may have enabled the Inca masons to make joints without any painstaking trial and error.

Back in Ollantaytambo, Vince is about to use his scribe

as he attempts to make a perfect Inca joint between two stones his masons have worked on for several days.

VINCENT LEE: We're getting the rock into position to scribe this prepared joint into this one that's yet to be prepared.

And so far, everything we've done, anyone fitting these two rocks together would have to do.

You would have to roughcut your rocks and basically decide which rock was going where.

And you would have to get them in position.

Now is the point where the method I'm proposing perhaps differs from others,

because what I'm saying is that by using this scribe,

this end, this blunt end is designed to work against a previously prepared smooth surface.

Now what we have to do is make this edge exactly match it.

And the way we do that is by taking this scribe and running it down this pre-finished surface,

maintaining the string hanging through the center of its hole with this little plumb bob

so that we don't accidentally mess up our joint by allowing the scribe to move in this plane.

As long as we keep the string in the center of the hole,

and as long as this is rubbing against that pre-finished surface,

all we have to do is chop this rock out so that this end of the scribe exactly fits-no matter where we put the scribe.

Then, we can achieve the fit we want by moving this rock one more time, simply closing the joint. End of story.

Time constraints have forced Vince's men to use steel chisels to work the hard andesite rock.

ED FRANQUEMONT: Well, this is it, the moment of truth for Vince's project.

He's been scribing and chomping and chipping and polishing.

And right now, these two stones are supposed to go together like Inca masonry, be right close together.

What do you think? Gonna work? Gonna happen?

Ed's absolutely right. It's time to stop talking and start moving rocks, so, so let's do it.

The joint that we've gotten is certainly not as good as the ones we've seen up in the ruins. But it isn't bad.

What we did here today is, we fitted two large rocks together, moving them together only one time.

That's the essence of my idea, basically. So, we didn't have to try this back and forth at all.