Be prepared to put any jaded tourist scepticism aside for a while when you inspect the sombre labyrinth of underground passages.

In 1955, when the bunker was constructed, the Cold War was a reality. Thousands of nuclear-tipped missiles were ready to be launched by the United States and the Soviet Union at a moment's notice.

As a policy of deterrence, Mutually Assured Destruction does seem insane now but that was a different era. The grey concrete corridors of the bunker are a reminder of a bleak chapter in the past, and now contrast strongly with the effervescent, cultured city above.

Prague, well known for its beautiful bridges and bustling Old Town, is the historic capital of Bohemia.  After World War I, it became the capital of Czechoslovakia.

During its 1100-year existence, the city has had a painful history, with occupation more recently by the Nazis in World War II and the invasion by Soviet tanks in 1968.

Many of the city's famous cultural attractions survived the violence, including the Prague Castle, the Charles Bridge, Old Town Square and the Jewish Quarter.

Tourists who flock to Wenceslas Square walk past the memorial to Jan Palach, a university student who burnt himself to death outside the National Museum to protest against the 1968 invasion which ended the "Prague Spring" reforms.

On the museum wall, bullet marks are still faintly visible, the result of troops shooting at what they thought was the broadcasting site of a freedom radio station.

In 1989, half a million people jammed the square to hear speeches supporting the Velvet Revolution which ended 41 years of Communist rule.

The balcony of the Melantrich building from where the speeches were delivered is still there but, perhaps fittingly, while the facade has been retained, the building has been extended to become a bastion of capitalism and now houses Marks & Spencer along with numerous other stores.

To capitalise on a niche market in this history-rich city, one tourist company is operating a "Communism and nuclear bunker" walking tour.

Our guide is Ondra who tells me his day job is answering calls on Prague's equivalent of the triple 0 emergency hotline.

He begins the tour by trying to compress four decades of Communist rule of his country into a 10 minute mini-lecture.

The quartet of Americans in the small group looks bored and stony-faced at the intricate details of Communist rule.

With the history over, the walk begins.

Ondra stops at an almost hidden memorial to the Velvet Revolution, a plaque on a wall where protesters were bashed. Workers, shoppers and tourists hustle by, oblivious to the historic site.

Next he halts outside a dark brick, abandoned building which housed the secret police. A row of adjacent houses were used for "interrogation", he says, summoning sufficient distaste into the phrase to convey the real purpose.

Indeed, one prominent activist died after eight hours of questioning. "You see, it was really torture," Ondra adds.

The tour of the bunker begins at a graffiti-covered wall with a door that, on closer examination, is one third of a meter thick.

You would have walked past without realising Prague's biggest bunker lies beneath except, of course, today you are on a mission, looking for the way underground.

Entering the bunker is truly spooky. It was built with the ultimate nightmare of a doomsday scenario in mind, where millions of people who stayed on the surface would have died.

But would those who shuffled into this world of concrete corridors have survived?

The bunker was originally intended for 5000 people. Really? We are seeing just the one third that is open and maintained but even so, the corridor seems crowed by just our small band of adventurers.

Ondra says the original design was to allow a half square meter for each person, as he uses his hands to shape a square around his feet.

Later, that provision was relaxed to a generous one square meter per person, perhaps after the rehearsal was held with a mere 400 volunteers.

After a real missile attack, the survivors were expected to remain underground for a month before being evacuated to rural areas.

Meanwhile, 16 meters below ground, the toilet doors in the bunker are made of wood, Ondra says, so they could be smashed in if it was feared the person in the cubicle was attempting suicide.

In the stunned silence that followed, he didn't need to hint further at the mental state of people herded together and told to wait for weeks, with absolutely nothing to do except worry about the food and water running out or being contaminated, before they finally emerged, to view Armageddon.

The filtration plant designed to scrub contaminated dust from the air looks primitive.

Next door is a display of gas masks and a reprinted user manual for Gasmask M-10 and instructions for putting on a protection suit.

Other Cold War era exhibitions have photographs and newspapers from back in the day.

Tourism kitsch is not overlooked. Visitors can dress up in an ancient army coat and cap, grab a rifle and pose awkwardly for a photo in front of the hammer and sickle flag of the former Soviet Union.

Emerging into the sunlight, the guide shepherds his flock onto a sleek, three-car tram that would look superb shuttling between Civic and Gungahin.

The tour ends back in the square but goes via an almost hidden garden, now open to locals after bring returned to the Franciscan monks after the end of Communist rule.

Ondra is not big on jokes but he does have a well-rehearsed punch line for the nine souls who have made the journey.

He reports that young people in the Czech Republic have very little knowledge of the days of Red Rule.

So, as he shakes hands with each person, ostensibly as a farewell, he is grinning goofily and congratulates the four Americans, two Germans, one Swiss, one South African and the lone Aussie, for now "knowing more about the Communist era" in his country than the typical young person walking by to get lunch.

www.prague-communism-tour.com

The writer travelled at his own expense.