Brooklyn urban spelunking mystery (opens in new tab)

Years ago in Brooklyn, a wine-fueled Friendsgiving led to discovering a mysterious secret cavern underneath a Brooklyn Heights historic mansion. What did I find? No, it's not the Atlantic Avenue tunnel. Looking for actual records, not speculation.Many years ago, my friend lived in the garden/ground level apartment at 150 Columbia Heights, right up against the Brooklyn Promenade in Brooklyn Heights. The door to enter her apartment shared a small vestibule with a different door that we assumed ...