This is Dimitrov's mausoleum. Dimitrov died in Moscow. Many Bulgarians believe he was poisoned by Stalin because Dimitrov and Tito were discussing a southern Slav state that would join Yugoslavia and Bulgaria.
After the fall of communism, Dimitrov's mausoleum became, according our guide, a rundown, stinking public urinal. Although the building was constructed in six days, it took the government four tries to destroy it. (Here's a clip of the unsuccessful third try.) Our guide claimed that the mausoleum was difficult to destroy because it had been designed to withstand a nuclear attack.
The decision to destroy the mausoleum remains controversial.