This is true.
An unidentified local man in Cappadocia, Turkey discovered an ancient, underground city in 1963.
While renovating his home, he discovered his chickens were disappearing into a gap in one of the walls. According to the New York Post, "In an effort to recapture his escaping poultry, the unidentified man knocked down the wall in the 1960s to reveal a dark tunnel leading to the ancient city of Elengubu, known today as Derinkuyu."
BBC stated the city, which is located 85m below the Earth's surface, is the largest excavated underground city in the world, with 18 levels of tunnels and the capacity to hold up to 20,000 inhabitants. (Eighty-five meters converts to about 279 ft.)
The exact date of the city's construction is "contested," but it is believed to have originated around the 8th century B.C.
The underground city is listed on the UNESCO World Heritage list in 1985 and can be toured.
According to Snopes.com, "Due to ongoing excavation efforts, the details surrounding the scope of Derinkuyu's discovery have not yet been fully revealed."
https://www.snopes.com/fact-check/man-unearths-underground-city-turkey/
https://www.architecturaldigest.com/story/inside-turkey-underground-cities
https://whc.unesco.org/en/list/?search=Derinkuyu&components=0&order=country
https://www.bbc.com/travel/article/20220810-derinkuyu-turkeys-underground-city-of-20000-people
https://www.businessinsider.com/abandoned-underground-city-derinkuyu-turkey-photos
https://nypost.com/2023/06/15/ancient-turkish-city-discovered-after-man-knocked-down-basement-wall/