Narrator: By the time he's 45 years old, Bartholdi has spent nine long years working on the statue.

Over and over again, work grinds to a halt.

But each time, he manages to revive it.

But one of his biggest challenges is how to build a statue so tall.

Fortunately, one of France's most accomplished engineers, Gustave Eiffel, has the answer to Bartholdi's problem.

Edward Berenson: A lot of Americans don't know that the skeleton of the Statue of Liberty, the thing that actually holds it up, was built by Gustave Eiffel, who later built, of course, the Eiffel Tower.

You could say that there's an Eiffel Tower inside the Statue of Liberty.

François Blanchetière: [English dub] Gustave Eiffel was establishing his reputation, particularly as a bridge builder.

He was becoming a major figure in the field of metal construction.

Narrator: Eiffel mastered a technique for building strong lattices of iron, which allow for immense load-bearing and wind resistant structures.

Eiffel's team gets to work modeling the forces that could affect the statue.

At over 300 feet tall and with more than 30 tons of copper needed for the exterior, the structure inside needs to be super strong.

It's only on the inside that one can see how it works.

Edward Berenson: One of Eiffel's great innovations was to figure out how to support the statue, not from the bottom, but actually from the top.

The exterior of the Statue of Liberty hangs on a series of supports, which is really quite extraordinary.

There is an internal column-- pylon-- made out of wrought iron, designed in the same way that Eiffel designed his bridges.

Narrator: At 92 feet tall, the pylon bears all the weight-- like the tower on a suspension bridge.

Then, a secondary metal framework radiates out from the tower to connect to the copper plate shell on the outside.

The copper was light enough for such a tall sculpture to stand upright.

A skin of copper that almost floated on this skelton of wrought iron.

What this system really does is it makes the Statue of Liberty float on what amounts to a series of springs.

Bertrand Lemoine: [Speaking French] [English dub] The system allows for a certain degree of mobility, a certain expansion.

The statue is exposed to the sun, so it deforms slightly.

It allows it to absorb the effects of the wind.

Edward Berenson: And it had to stand in New York Harbor, where there can be hurricanes.

As a-- a work of engineering, it's really extraordinary.