A mystery at the Laurentian Library
Within the Laurentian Library, the enigmatic masterwork of Michelangelo, there exists a complex geometric pavement that is hidden from view, little known about and shrouded with mystery. The Library is situated within the Medici family church complex of San Lorenzo in Florence, and like many Renaissance libraries, it is located on the upper level of a cloistered courtyard. Once the upper level is reached, there is little fanfare announcing the Library entrance and it would be easy to walk by it unless you knew where it was located. But on passing through the door, quite a different world opens up; the visitor stands in the corner of a gigantic room that ominously stares downwards, leaning on the senses with its architectonic weight ...
The pavement appears to have been intentionally concealed for centures - this site tries to determine why. More information about the library's history is here. Just for good measure, there are some cool 3D models of the library available here.