rafael tbvmentuastavino and his son brought an old worl masonry technique. >> it was fire proof, it was incredibly strong. and it could be built with no support from below during construction. almost like magic. >> we're right between the outer dome and the inner dome. unbelievably thin, four inches thick. >> just four inches? >> yep, remarkably thin. >> yardagely forgotten by the -- largely forgotten by the public. they are getting their due. >> lost to the public. john oxendorf is the cure parity. scale model appears to defy logic. no support from below but somehow it can hold tens of thousands of pounds. >> feels solid. >> very solid. the key is getting the geometry right. and they were masters at getting geometries that were very strong. and so that was what they brought from the old world. >> their designs weren't just strong, they were also beautiful. but few people have ever seen their masterpiece. city hall subway station hidden in downtown manhattan is often referred to as the mona lisa of subway stations. >> when it was first opened it was called an under ground cathedral. sky