Mr. Valkov, 24, the founder of Swift Gap Events, a small Sofia-based events management company, chanced upon it while scouting performance spaces last fall.

“From the outside, this looks like just another communist-era building,” Mr. Valkov said. “But when I entered inside, I was like, Wow, we need to organize an event here.”

By May, the facility was the site of the Balkan Beats Festival, which featured an eclectic array of music, from Berlin-based D.J.s to a band that fused jazz with Bulgarian folk music. Hundreds of people danced in the empty pool, while acts emerged through smoke to take the stage as blue, red and purple lights brightened up the arena.

Mr. Valkov has since called the site the “Underpool Arena.”

Several other arenas in Sofia are trying to undergo similar transformations from communist-era afterthoughts to trendy present-day cultural sites.

Half of a former wholesale food market, for example, is now home to a collective of young artists.

When the market opened in 1985, cargo trains would unload tons of food supplies into its yard. Once big supermarket chains entered the country, however, the market gradually lost relevance. Now, some companies use the ground floor as a storage depot, but the rest of the seven-story building had been empty for years.

It was taken over in 2014 by Culture Lab, which rented the vast area to house a diverse array of artists, creative workshops, galleries and Sofia’s only indoor skate park.

Visitors to the building are greeted by a giant white sign reading “Keep calm and rule the world” painted in English on the scarlet red iron doors. Nineteen groups of artists now occupy the space, including Chudomir Dragnev and his metalworks studio, Looneytools.