This formation has been recently reimaged:

 A shows the unusual bulls eye shape, hard to explain geologically. In fact this seems to be the only on of its kind on Mars so far. B shows an area that appears to have had water and ice. These possible artificial formations often seem associated with water erosion making them likely extremely old. C, D, and E show a possible breakwater or dyke, inside this line has different terrain to outside it. F and G show a second dyke like formation. This is similar to another possibly artificial formation here:

In the next image A shows the same tunnel like formation leading into the amphitheatre as on the previous page.