(ed. note: Not really secret. Also, not actually the Batcave.)
From the DAILY NEWS: "Within the next two months, construction will begin on an underground auditorium that was planned when the Grand Army Plaza Library was built in 1912 … ‘The hole for the auditorium was dug,’ Ginnie Cooper, executive director of Brooklyn Public Library, said. ‘They poured the concrete for the entire building – and then they stopped.’"
"… the $16 million project … also includes a new plaza … The auditorium will seat 200 and will have a sloped floor, a fixed stage and a video screen.
Cooper envisions conversations with authors, panel discussions and small music presentations … The entire project will take 18 months to complete…" Read it…
Also, check out this amazing construction photo from the 1910s, courtesy of our new best friends at the Brooklyn Public Library.