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

In the year 1623, Christian the 4th, King of Denmark and Norway,

built a long series of mootes and ramparts just across from

central Copenhagen on the eastern edge of the city's harbor. For much of the

17th century the fortification protected the city from Swedish invasion. In the early

19th century the Danish government added artillery barracks and in the 20th century

they filled in the surrounding swamps to make room for a modern military base, even as the rest of Copenhagen grew around it.

That is, until 1971, when the sprawling fortress in the center of the city transformed into something else entirely.

And he started up with the kids who had of course been wondering what goes on in there.

The show the soldiers coming in and out,

they heard the military music,

but they were never allowed to go in there.

John Bancalson was in his early 20s

when he moved to Copenhagen from his conservative home in the Danish

countryside. And he says the symbolism of a military base in the heart of the country's capital

was not lost on him and his friends.

As reporter, Scott Guryan.

The base's walls represented authority,

repression.

Pretty much everything Denmark's younger generation had come to Copenhagen to escape. But in 1971, the Danish

defense ministry decided they no longer needed the base and they closed it for good.

So suddenly when one morning we see all the soldiers marching out of

...