A fire has broken out at the retreat built for Adolf Hitler in the Bavarian Alps known as the Eagle's Nest, and a person has been detained in connection with the blaze.
A fire alarm at the building in the Berchtesgaden district in Germany's southeastern corner went off on Friday fright, police said.
Firefighters who went to the scene could see the flames as they approached the site, on a cliff about 1800 metres above sea level.
Getting fire engines to the site proved difficult because of its location, and work was still ongoing to put out the fire on Saturday morning, police said.
The building and its immediate surroundings could only be accessed with respiratory protection equipment.
Firefighters found no one in the building and authorities believed that no one was injured.
Police said a person was detained as a possible suspect, but it has yet to be determined whether they had anything to do with the fire.
The building, the Kehlsteinhaus, was completed in 1938 and is reachable by an elevator from the end of a winding road up the mountain, which is closed to public traffic but used by buses for visitors.
It has been operated as a cafe and restaurant since 1952 and contains an exhibition on its history.