Stairs.

Also, in the previous post there was reference to Esher and Piranesi - these images are (stair related) examples of their artworks:

Esher:

Piranesi:
Referencing:

Both of these images were collected off the internet from the following sites, however they are artworks by the artists mentioned(Esher and Piranesi). Both these materials are commonly available, however I got them from the links below.

Maurits Cornelis Escher (1898-1972):

http://www.mcescher.com/Biography/lw435f2.jpg

Giovanni Battista Piranesi (1720 - 1778):

http://dl.lib.brown.edu/melancholy/spaces.html