Manhole covers from the 21st century?
Street "traps" or elevators are definitely an interesting feature of cities that I am noticing lately (Paris and Lyon above, Geneva below).
These devices definitely remind me of the pipes in Mario Bros, sort of tubes that allow people to be transferred to some underground secret world:
Beyond this aesthetic concern, they do exemplify the 3D nature of cities. More importantly, this hidden stairs/elevators are important as they reveal the underlying infrastructure of the city as well as the need to access this infrastructure (sewage, electrical station, etc.) to fix things. It's the sort of modern version of manhole covers in a more technological fashion.