This intriguing photos is the so-called "port," which has been examined in some detail at Mars Unearthed (no longer online).
This highly geometric structure, appearing on a Martian cliffside, looks like an artificially constructed two-story building of some unknown purpose. In the opinion of the article, "the first floor walls of [the port] are facing camera and in shadow. The second floor is a well-defined square and is turned at a precise 45-degree angle to the first floor. Centered in the midst of the roof of that squared second floor is an abrupt, sharp circle... a 'landing pad' as for helicopters atop tall buildings and on decks of ships."
Near this structure, the article continues, is a large, open, U-shaped culvert or conduit out of which is flowing a great deal of liquid.
