Lombard Street is San Francisco-- and America's crookedest street.  What does this mean?  The steep, hilly street was created with sharp curves to switchback down the one-way hill past beautiful Victorian mansions.  The street is paved with bricks and is an amazing site to see as demonstrated by the photos below.

If not for the byzantine curves, easing out this treacherous slope, people would be killed rolling down. For an idea of how steep this street really is, go two blocks up, to Filbert Street and peer down over the ridge. Lombard is even steeper.