The Viru Hotel, built in 1972 as Tallinn's main Intourist hotel for foreign visitors, concealed a secret KGB surveillance operation on its hidden 23rd floor for the entire Soviet period. The floor was inaccessible to hotel guests and staff, and it was here that the KGB monitored conversations, managed informers, and processed intelligence from the hotel below. When the Soviets withdrew, they left the equipment largely in place.
The museum is small — just the one room and a guided tour — but the guide is excellent and the story is told well. It covers not just the specific operations at the Viru but the broader mechanics of Soviet surveillance in occupied Estonia. Guided tours only; book in advance through the hotel.