Marvel's Agents of S.H.I.E.L.D. left mixed reviews for its first season, but it has been making some changes in Season 2 that have left its viewers wondering.  One of the subplots has been given a little light as it was revealed that Agent Coulson (Clark Gregg) has been drawing a city the whole time with his odd habit of making drawings. 

This revelation happened on episode 7, when Coulson and Hank (Joel Gretsch) were kidnapped by Sebastian (Brian Van Holt).  Sebastian had all kinds of tattoos that looked like circuitry all over him, which bore a strange resemblance to the writing that Coulson had been doing the entire season. 

Coulson and Hank were held in a barn that had this weird multi-level train set that Hank had set up.  When Coulson sees the train set, it is enough to make him realize that the drawings was "not so much a map as a blueprint".  This blueprint is to be some sort of city. 

Coulson then tells his team that their job is to "find it before anyone else".  Of course, there is a lot of speculation of the alien city, and episode 8 of Season 2, "The Things We Bury" could lead to more clues, or more mystery. 

According to a sneak preview, an episode will feature Agent Peggy Carter, who is played by Hayley Atwell, the same actress playing the same character from Captain America: The First Avenger.  Carter is interrogating a man in the 1940s named Werner Reinhardt, who is also known by Coulson as Daniel Whitehall (Reed Diamond).  The interrogation room is in an SSR (Strategic Scientific Reserve), and the two characters are discussing artifacts that the SSR recovered from Hydra.  At this point, Reinhardt tells Carter of visitors that come down from the sky. 

Reinhart also states that "these visitors, they didn't come to save the world...they came to conquer it."

So, who are these visitors?  It could easily be the Chitauri, who we saw a lot of in the last Avengers film.  They were kind of underdeveloped henchmen that kind of went down quickly after the big threat of the mothership was defeated, and they could use a little more flushing out in Marvel's cinematic universe.  Is the city supposed to be a new mothership?

Of course, Marvel has many alien races that it can pick and chose to fill the role of villains.  The Kree were visible in last summer's Guardians of the Galaxy, and it is not known how Guardians will fit in with Agents like it has with the other Avengers-related films.  The shape-shifting aliens The Skrulls could be another possibility, and it certainly create a lot of interesting twists like Marvel's Secret Invasion storyline in the comic books a few years ago. 

There is one option that would fit well in this Marvel cinematic universe.  Since Marvel recently announced their latest cinematic offerings would include an Inhumans movie to be released in 2018, this alien city plan could be the introduction of Attilan.  Attilan is the ancestral home of the Inhumans, who are a more advanced version of the human race.  Attilan was originally an island out in the middle of the ocean, and legends eventually called it Atlantis.  After it was moved from the middle of the sea, it was then moved to the Himalayas, and then to the moon to escape Earth's pollution and disease.  The Kree aliens once created a city on the moon and gave it a "Blue Area" that allows for an artificial atmosphere. 

So, will Coulson and his team find Attilan?  Considering that the Inhumans movie is four years away, it would be awkward if they found it this season and didn't develop it until the Inhumans movie.  It might be enough to keep viewers engaged, and make viewers wonder more about the long-range plan of the Marvel Cinematic universe. 

Catch Marvel's Agents of S.H.I.E.L.D. on Tuesdays at 9/8 on ABC.