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

Five steps to keep in a defecting population.

Step one.

Build a 12-foot reinforced concrete wall.

There was a wall.

Step two, build a second wall right next to it, creating a no man's land in between.

Then there was a 10, 20 meter wide strip of land which there was absolutely nothing.

That was sprayed with chemicals that not even grass could grow.

And it was raked every day that you could see the slightest clue

that there was an escape.

Step three, build a narrow road for vehicles of the secret police.

Di Stasi.

Step 4, add electrified barbed wire, and also.

Step 5, signal wire.

Signal wires that if you touch it immediately in one of the watchtowers, there was the alarm ringing.

At its peak, the Berlin Wall was 100 miles long. Today only about a mile is left

standing. Compared with other famous walls in history, this wall had a pretty short

lifespan. The Great Wall of China has been around for 2,500 years.

So have the walls of ancient Babylon. Although its most famous part, the Ishtar Gate is actually

in a museum in Berlin.

This is Daniel Gross.

But even though the wall dividing Berlin into East and West

...