Sorry for long delay - Christmas happened and I've been dealing with shingles, lucky me! The Bramante Staircase in the Vatican - a double helix staircase so you can either ascend or descend and not pass someone going in the other direction on the same side.

(Also if anyone knows who took the original photo so I can credit them please let me know - I can't find them through any of the image sites I know and Google Lens searching isn't helping me either.)