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This is 99% invisible. I'm Roman Mars.

I want to be careful not to overstate when it means for a building's worth is an infinitesimal fraction of the worth of a person's life.

Even two buildings don't even move the needle in comparison to real human loss.

But a building is still a living thing in a way.

It breathes and it moves. This movement makes a sound. The structural engineer who designed the World Trade Center towers says that the people working inside couldn't feel this movement of the towers swaying in the breeze.

But they could hear it. Often,

often,

often,

often buildings speak to you.

What happens in a tall building is that in the wind

as the building moves, the floor above moves with respect to the floor below.

My full name is Leslie E Robertson.

I'm a structural engineer and I was responsible for the design of the World Trade Center.

This cassette tape was one of many that we took during the construction and

later occupation in the building.

For each cycle of oscillation of the building,

...