An Old Slurry Pit
The secret underground room used to function as a slurry pit; in other words, a dam used by farmers to gather animal waste. Now, it serves as a home cinema and music room.
“The man who lives in the apartment plays guitar, so the downstairs room will be a music box and home cinema,” Rolland said in an interview. “That way, he can make music and watch films without annoying his neighbors.”