BERLIN (Reuters) - Three vast tunnels were opened under central Berlin this month, giving a glimpse of Adolf Hitler’s megalomaniac vision of a new architectural centre for the capital of Nazi Germany.

The dome of the 'Great Hall' is pictured at the exhibition 'Myth Germania' in Berlin March 11, 2008. REUTERS/Johannes Eisele

The 16-metre (50-foot) deep tunnels were constructed in 1938 as part of an underground transport network beneath a series of bombastic buildings designed by Nazi architect Albert Speer, including the biggest domed hall the world had ever seen.

The overground plans, never completed because of World War Two, included boulevards, squares and huge buildings, such as an arch dwarfing the Arc de Triomphe in Paris, and the 290-metre high Great Hall, with room for 180,000 people.

Hitler called the concept, a symbol of the power of the Third Reich, “Berlin -- the capital of the world” but in recent times it has come to be known as “Germania.”

The tunnels, between 90 and 220 meters long lying beneath the Tiergarten park, would have accommodated roads and a railway line.

“The tunnels -- which are in surprisingly good condition -- were part of Speer’s grand plans, what we now call ‘Germania’,” historian Dietmar Arnold, head of the Berlin Underground Association and bunker tour guide, told Reuters.

Last week, Arnold -- who runs an exhibition of Hitler’s plans -- took journalists on a rare visit into the dank tunnels.

They are closed to the public most of the time because of safety concerns, but visits can be arranged.

“The acoustics are incredible,” said Arnold, who likes singing a note and hearing it reverberate around him.

After the war, British forces in divided Berlin closed the tunnels. They were rediscovered in 1969 but have remained shut. In 1990, a year after the fall of the Berlin Wall, they were handed to the city of Berlin.

The Berlin Underground Association, set up in 1997, has seen a surge in interest in tours of Berlin’s remaining bunkers.

Although most were destroyed, some of the maze of 1,000 World War Two bunkers are intact and serve as a reminder of the city’s violent history.

Propaganda posters and escape instructions on the walls convey a sense of the past. In one bunker, suitcases, helmets, and uniforms from various sites are on show.

“Interest is constantly growing -- we have about 150,000 visitors a year to the bunkers,” said Arnold. “That is partly why we want the bunkers to be protected -- they are an important part of the history of Berlin.”

By the end of the war, Germany’s most heavily bombed city could protect up to 800,000 people in its bunkers.