This is the boarded-up exit to the basement below the skeleton of the first steel-framed conservatory in America, on the grounds of Lyndhurst, a Gothic Revival mansion.
Helen Gould, the wife of robber baron Jay Gould, who damn near cornered the gold market in September of 1869, used to stock the conservatory with Night-blooming Cereus.
Night-blooming Cereus is a cactus that only blooms one night a year. Yeah: night.