In short, this spiral staircase was built for Sante Fe NM nuns by an unknown mysterious carpenter. No nails, no support means, just pegs. They attributed this to something miraculous. Snopes says no, but whichever way you go, I think it's an interesting story.
http://www.evanderputten.org/special/ne ... oretto.htm
http://www.csicop.org/si/9811/i-files.html
http://www.snopes.com/horrors/ghosts/loretto.asp