The Loretto Chapel spiral staircase. The story goes that the nun’s needed a staircase and prayed to St. Joseph, patron saint of Carpenter’s, when a mysterious man came and offered to help construct a staircase over the next couple months, but he disappeared without receiving pay or thanks and could not be found. Within the staircase there is not any support or nails to be found and the staircase makes two perfect 360 degree turns.
In any event, the finished staircase was an impressive work of carpentry, seeming to defy physics as it ascended 20 feet without any obvious means of support. The staircase as originally built lacked handrails and was reportedly so frightening to descend that some of the nuns and students did so on their hands and knees. Eventually, railings were added in 1887 by another craftsman.
In reality, the staircase is supported by its stringers just like a conventional (straight) staircase. In addition the assembly of the stringers from overlapping segments joined by wood glue creates a laminate that is actually stronger than the wood alone. (Source & Source)
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