- [Narrator] The Statue of Liberty.

Under its skin lies one of the secrets to the Eiffel Tower's structural strength.

In 1870, renowned French sculptor Auguste Bartholdi imagines a 300-foot high statue in the form of a woman, celebrating the signing of the U.S.

Declaration of Independence.

- Bartholdi began his thoughts about the Statue of Liberty in Egypt.

He was making terracotta, little models, but when it's about realizing he has to turn sculpture into a modern phenomenon, the reason it can be that gigantic is that it's hollow.

(upbeat music) - [Narrator] Tall and in the shape of a person, yet hollow.

How does this structure hold together?

It's 6:00 AM in New York City.

Before thousands of visitors arrive, ranger Matt Housch leads the way on an exclusive tour into the heart of the statue.

The similarities with the Eiffel Tower are easy to spot.

(upbeat music) - [Matt] What's most impressive about the interior of the Statue of Liberty is how all of this iron and steel works together to hold her over 300 feet above New York Harbor, over a hundred years of wind and rain.

And she still stands because of this interior structure.

- [Narrator] After the teams riveted together the internal structure's iron beams, they next installed a secondary structure, made out of hundreds of iron bars.

On top of these bars, they attach the copper skin piece by piece.

And the secret to these layers holding together is in one simple but incredibly effective solution: rivets.

- The inside of the Statue of Liberty can be a disorienting place, but what you're seeing are hundreds of copper plates.

So that's the dark metal that you see all along the interior here.

That's her skin.

And those copper plates were all riveted together with thousands of little copper rivets.

But the copper skin has to be held up.

So we can see there are thousands of steel bars connect the copper plates to the secondary iron bars, and all of those iron bars connect back here to these iron pylon.

(upbeat music) - [Narrator] Building, high, building light, and building strong, mastering the art of riveting for the Statue of Liberty would prove crucial for years to come.

(upbeat music)