Two people can climb this staircase without ever crossing paths. Inside France’s Château de Chambord, a remarkable 16th-century staircase consists of two separate spiral stairways winding around the same central axis. People using different stairways can ascend or descend simultaneously without crossing each other’s paths. They can even see one another through openings in the structure, despite remaining on separate staircases. Built during the reign of King Francis I, the double-helix staircase is one of the château’s most distinctive architectural features. Its design has long been associated with Leonardo da Vinci, although his direct involvement has never been conclusively established.