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.

Das nie man's land. 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 this time.

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

Distazi. Step four. Add electrified barbed wire. And also step five.

Signal wires. 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 a hundred 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 2500 years.

So have the walls of ancient Babylon. Although it's 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 was only up for 30 years, it had a huge impact on the psyche of the city.

It broke families in two.

Now let's remember how it got here. In 1945 Berlin was the fallen Nazi capital.

The weary victors could agree on two things. One, Hitler was bad. Two Germany needed a big change.

After that they did not agree on very much.

Berlin was carved up into two sectors, with Western countries controlling the west of the city and the Soviet Union controlling the east.

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