Double Helix Spiral Staircase - Vatican Museum
The beautiful spiral staircase at the Vatican Museum, from below. Designed by Giuseppe Momo in 1932, the broad steps are somewhere between a ramp and a staircase. They were designed to be wide and have a small enough amount of climb so that horses could walk up them. The stairs are actually two separate helixes, one leading up and the other leading down, that twist together in a double helix formation. I think it's really cool that this was designed many years before the discovery of the double helical DNA strand, which has now come to represent life itself.