Renwick Hospital, Roosevelt Island, New York City

This abandoned Smallpox Hospital, replete with granite veneer, corbelled parapets, and mansard roofs, is a reminder of Gotham’s grisly past. Its 100 hospital beds once hosted quarantined immigrants suffering from the gruesome disease. An ongoing $4.5 million restoration project will open Renwick to the public in 2013, kicking off with an art project that includes giant butterflies hovering over the site.

Mystery: Renwick is currently illuminated at night by an anonymous patron, who purportedly has a view of it from an Upper East Side penthouse.

Visit: The American Institute of Architects and Classic Harbor Line offer architecture-themed cruises around Manhattan with lectures on Renwick and other mysterious city sites

Unusual Buildings