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It took me a bit to realize it’s actually a double helix. One spiral with people going up and the other with people going down.
Here [are](http://upload.wikimedia.org/wikipedia/commons/5/5b/Vatican_Museums_Spiral_Staircase_2012.jpg) [a](http://www.flickr.com/photos/vintzileos/482275453/) [few](http://www.flickr.com/photos/davaodude/5515263496/in/photostream/) [more](http://www.flickr.com/photos/nathangibbs/202549889/) [images](http://www.flickr.com/photos/ogil/1383346804/)
Apparently it was designed to be so wide and with such low slope to accommodate horses.^(**)