Looking up the International Market Square building stairwell in Minneapolis, Minnesota. This is the largest commercial double helix staircase in the US - built with straight runs and flat landings it intertwines two separate sets of stairs, one for going up and one for going down.
More than you probably want to know:
Although it looks like artwork by M. C. Escher, these unique stairs have a more utilitarian reason for existence - or so the story goes: International Market Square used to be the old Munsingwear underwear factory. George Munsing invented non-scratchy underwear in 1891 which was a miracle for the textile industry but sewing the stuff all day was not a fun job and employee morale became a problem. When George designed his new state-of-the-art factory in 1904 he spared no expense by putting in one set of stairs to accommodate happy people coming to work, and one set of stairs for the exhausted people leaving. With two sets of stairs, different shifts of employees would never run into each other - morale problem solved.
The Munsingwear company was extremely successful and became the largest manufacture and distributor of underwear in the world. Eventually competition caught up and market declines forced Munsingwear to close down the Minneapolis building in 1981. In 1985, the old factory was renovated and the complex was renamed International Market Square, which currently holds offices, shops, showrooms and this cool set of stairs.