From a reddit post I found:

The complex was lost and forgotten for centuries; then found by accident by a local pilot in 2000.

On a clear February morning in 2000, Nicolino Lombardi, a local historian and pilot, flew his ultralight over nearby Mt. San Nicola. He planned to photograph a recent landslide on the hills south of his village of Pietravairano in the Campania region of Italy. Lombardi had photographed this region in the past and was accustomed to seeing fragments of ancient Samnite, Roman, and medieval ruins mostly obscured by blankets of foliage. However, during this particular flight, the view was different. A recent brushfire had cleared the overgrowth and revealed a long-hidden treasure: the ruins of a theatre/temple complex. In his report following his discovery, Lombardi wrote, “This time, however, the symmetry immediately catches the eye, and everything is clear in a flash: I even seem to see a temple and a theater, with the colonnade still standing and the wide steps full of spectators” (Lombardi p.1).

His photo from 2000

More info (in Italian)