One of the most fascinating images, pictured at right, shows a ribbed tube or tunnel-like structure. Partially covered by the surface terrain, the structure looks as through it has been unearthed (unmarsed?) by some geologic process. The structure resembles the so-called "glass worms" discovered in other photos, although this one lacks the "transparent" or "translucent" quality of the worms.
The official NASA stance on these structures is that they are quite natural and are, in fact, "dune trains" - rows of sand dunes. The article "Is This an Artificial Construct on Mars" compares this photo with another photo of actual Martian dune trains. They don't look anything alike. While the photo of "actual" dunes indeed look like sand dunes, this enigmatic structure does not.