"The Buzludzha Monument, one of the symbols of the socialism in Bulgaria. The building is now abandoned but still stands alone in the most beautiful landscape".
Read more and see some amazing photographs at Artificial Owl and various other sites such as this one (more shots of the inside, sure to appeal to those with a taste for vintage Soviet science fiction films).
You might also enjoy a further selection of Communist Political Monuments in Bulgaria, which includes this beauty:-
Then be prepared to lose an hour browsing the rest of Artificial Owl for further collections of "the most fascinating abandoned man-made creations".