PRAGUE –  One thing was for sure when foreigners stayed at a prestigious Prague hotel during the Cold War era — their telephone conversations were carefully monitored by secret police in a hidden underground bunker some 20 meters (66 feet) under the building.

The Jalta hotel at Wencaslas Square in the heart of the Czech capital was built in 1958. Its massive bunker with its reinforced concrete walls was meant to provide Communist Party members and military officials a shelter in the case of a nuclear attack.

But it was also used as a center for surveillance operations that targeted western visitors staying at one of the several international hotels in Prague at the time.

To mark its 55th anniversary, the 500 square meter (5,382 square feet) bunker has since been turned into a museum. It opened to the public last week.

Sandra Zouzalova, Jalta’s public relations manager, said Wednesday the hotel wanted to shine a light on the many secret activities of the Cold War era.

Jalta was one of many places used by foreign diplomats where the Communists gathered intelligence. West Germany’s business representation office in the 1970s was one of the prime targets, she said.

“They were eavesdropping on all of the hotel rooms,” Zouzalova said.

Inside the bunker is some of the original equipment, including a switchboard, a tape recorder and numerous wires that once led to the hotel’s 94 rooms.

Also on display is a floor plan that shows some rooms painted in red, green and yellow.

Zouzalova said the red rooms were given to high-value targets.

She said the operation didn’t cover just phone calls.

Listening devices were attached to lint brushes, and prostitutes were often used.

The shelter, which had walls that were two meters (6.6 feet) thick, had its own ventilation system and a huge water tank that would allow more than 150 people to survive for months.

The place was shrouded in secrecy until 1998, when Czech authorities gave up the space for use. That was nine years after the 1989 Velvet Revolution that ended Communist regime.

The bunker is open two days a week and guests can visit with advanced booking.