On the surface, the Marvel Cinematic Universe is a place of wild adventures, bombastic fight scenes, and comic lightness.

Although each film has some version of a sinister plot threatening to dismantle the universe, the tone throughout the franchise stays light and fun. Audiences aren’t really filled with dread, as we know that our heroes will conquer the villains in the end, regardless of how long that might take.

While more serious films have come along, with some aspects of realism, like Captain America: Winter Soldier, the majority of the Marvel movies rely on noble characters and witty one-liners to keep their audiences interested.

However, when you begin to peel back the colorful layers of the Marvel Cinematic Universe (MCU), you begin to see that the world these films create isn’t all fun and games and Stan Lee cameos. There are a few darker truths and disturbing behind-the-scenes backstories to the Marvel films that are rarely discussed.

Here are the 15 Facts Even Die-Hard Fans Didn’t Know About The MCU.

A week of earthly catastrophes

Greatest Superhero Films Iron Man

After Marvel released its official timeline for the Marvel Cinematic Universe, fans were overjoyed at the ability to see clearly how the films were organized. With the timeline, connections between films could more easily be drawn to see how each movie influences the storyline of the next in this complex world that Marvel has created.

The unexpectedly dark aspect of this timeline, which either Marvel didn’t notice or didn’t care to change, was the reveal that the events of Iron Man 2, Incredible Hulk, and Thor all happen in the exact same week within the universe.

This means that three separate locations on Earth faced the destructive wrath of an Asgardian Gort, Whiplash, and a Hulk-army chase all at the same time.

While there’s always some collateral damage when it comes to superheroes battling against an oppressive evil, the collateral damage doesn’t often involve three separate cities virtually exploding at the same time, making this particular week in the history of the MCU a little less exciting and a little more tragic.

A reference to a horrifying universe

In Guardians of the Galaxy Vol. 2, the Guardians must fight a horrific, tentacle creature in the first few minutes of the film. The creature is referenced as a multi-dimensioned being called an Abilisk.

The Abilisk has sharp teeth, enormous bulging eyes on the side of its head, and a Kraken-like body. Although many people passed off this creature as just a typical dangerous space monster wreaking havoc, many superfans believe it is a reference to a much more sinister storyline within Guardians lore.

In the comics, a disturbing parallel dimension exists in which nothing dies. Instead, the universe consumes beings and churns them back out as mutant, corrupted creatures with tentacles and rows of teeth, not unlike the Abilisk featured in the film.

Some fans wonder if the dark Cancerverse will be making an appearance in future installments of the Guardians franchise.

Bruce Banner’s deleted suicide attempt

Hulk vs Bruce Banner (Mark Ruffalo)

In The Avengers, Mark Ruffalo’s Bruce Banner reveals to his fellow Avengers that, at one point in his past, he tried to kill himself by “putting a bullet in [his] mouth” when he couldn’t find an escape from turning into the Hulk.

While this is one of the rare sobering moments in the Marvel Cinematic Universe, it’s actually a reference to a much darker scene in 2008’s The Incredible Hulk.

At the beginning of this film, Banner, this time played by Edward Norton, loads a gun with the intention of shooting himself in the head, but turns into the Incredible Hulk and crushes the gun in his hand before he can press the trigger.

Presumably because of the dark thematic material of this clip in a PG-13 movie, the scene was cut but can now be found as a deleted scene on Youtube.

Iron Man never wears his suit

Tony Stark in his original Iron Man suit in "Iron Man"

The Iron Man mecha-suit has become a steadfast symbol of the Marvel Cinematic Universe, so you might be surprised to learn that actor of Tony Stark, Robert Downey Jr., has not actually worn a full-bodied version of the suit since the first Iron Man movie in 2008.

According to visual effects artist Marc Chu, this is due to the fact that the Iron Man suit is heavy and incredibly uncomfortable.

Chu has state that, after Downey Jr. saw Iron Man and witnessed what CGI artists were able to do with their digital renderings of his suit, he realized that it wasn’t necessary for him to wear the full suit anymore during filming.

Every time you’ve seen Tony Stark in his suit after the first Iron Man, he is either wearing a CGI suit, is a stunt person, or is in a “football suit” version of the suit, which is just a headpiece and metallic shoulder pads, which is mainly used for close-ups.

The Slither slugs cameo

Unless you have the attention to detail of a skilled serial killer, you probably didn’t notice the slugs which briefly appear sliming their way up a glass tank in the lair of the Collector in Guardians of the Galaxy.

At first glance they may seem like a typical mock-up of a space worm-- a pretty unassuming addition to this collection of creatures-- but they are actually a direct reference to a film called Slither.

This horror-comedy hybrid was also directed by Guardians director James Gunn. The film served as Gunn’s first big feature, and due to its success, he was afforded the career that has led him to the helm of Guardians.

In Slither, these slimy slugs arrived from a distant planet to Earth via a meteorite, and work as fatal parasites, turning a small town into mutant zombie monsters. While some brush off the space slugs as just a fun Easter egg to Gunn’s past work, other theorists believe that these slugs, and the planet they come from, could possibly be worked into the plot of a future Guardians or MCU film.

Groot’s charming disease

Groot (Vin Diesel) in Guardians of the Galaxy

After the first Guardians of the Galaxy film, audiences worldwide immediately fell in love with the endearingly silent Groot and his simple catchphrase. Almost overnight after the first feature release, the sentence “I am Groot” was plastered all over Twitter and repeated relentlessly by every middle school boy in America.

While Groot’s only line was often used as a comedic punchline in Guardians, the reason behind this quirk is actually pretty disturbing. According to Groot’s origin story, Groot used to be able to speak in full sentences without a problem.

However, Groot and the rest of his species, all suffer from a unique disorder which causes their vocal cords to tighten and harden over time until all they are able to say is three simple words: “I am Groot.” So much for a cute one-liner.

Roxxon Corporation’s sinister cameos

Unless you’re a serial killer in an off season with way too much time on your hands, you probably didn’t notice that the brand name “Roxxon” turns up a lot in the Iron Man franchise.

In the first film, a neon sign saying “Roxxon Corporation” appears decorating a building in the background of the final fight scene. In Iron Man 2, the Roxxon Oil Corporation sponsors one of Tony Stark’s rival drivers in the 2011 Monaco Grand Prix. Finally, in Iron Man 3 the Roxxon Oil Corporation is involved in the oil spill and final confrontation at the climax of the movie.

While this may not seem like a big deal for the plot, the fun fact becomes a bit darker when you realize that the Roxxon Corporation has a dark origin story in the Marvel universe, often involving corruption and greed. In fact, they are even responsible for killing Tony Stark’s parents, which makes the appearance of the names much more than mere coincidence.

The corruption of the Roxxon Corporation is explored a little more in the Agent Carter and Daredevil series.

Obadiah Stane’s musical confession of evil

Tony Stark Obadiah Stane Iron Man

Unless you are a music history aficionado, it’s highly probable that you didn’t recognize the music Obadiah Stane plays halfway through the first Iron Man film. While Stane is telling Stark that he has been locked out of Stark Industries, he is casually playing a piece known as "Larghetto", composed by Antonio Salieri.

Salieri is remembered in history as the bitter rival and enemy of Wolfgang Mozart, and many believe that he even went as far as plotting Mozart’s death.

While Stane is playing this melody, the film subtly makes a comparison between Stane’s own darkness and jealousy and Salieri’s. Obadiah is the Salieri to Tony Stark’s Mozart, a rival of the charming and brilliant Tony Stark who attracts more fame and attention than he could.

This foreshadows the reveal of Stane’s sinister motives, which become clear as the film progresses.

Chris Evans didn’t want the part

Captain America Chris Evans Marvel

Many fans might imagine that actors would jump for joy and trample each other to a pulp for the chance to play a superhero in the Marvel universe. Because of this, you might be surprised to learn that Christ Evans turned down an offer to play Captain America, not once, not twice, but three times.

Due to his past experiences playing the Human Torch in Marvel’s Fantastic Four franchise, Evans knew what it was like to be a part of a high-budget film with less than favorable reception.

Because his original contract would sign him for nine films, Evans reportedly turned it down out of fear that he would be stuck making “spotty” films for an eternity. “I remember telling a buddy of mine, ‘If the movie bombs I’m f***ed. If the movie hits, I’m f***ed,'” he said.

Eventually Evans agreed to accept the part when he was able to negotiate his contract down to six films and got over his fears.

The Guardians of the Galaxy kidnapped shirt

While Guardians of the Galaxy has probably the most self aware, comically-driven tone of any film in the Marvel Cinematic Universe so far, it begins on a somber note, with a young Peter Quill saying goodbye to his mother who is dying of cancer. In this scene Quill can be seen wearing a green shirt which reads “River Raft: J.D. Canoe Rentals Dolores River.”

After stumbling out of the hospital in tears, Quill is abducted from Earth in a split second when he is beamed up into a spaceship.

Years later in the present, the t-shirt makes a surprising and unexplained return. It can be found on Bereet, a Krylorian who has a brief love affair with Quill in the first movie. The purpose of her wearing Quill’s kidnappee shirt is never really explained or addressed by Quill in the film.

A foreboding mystery cameo

During Captain America: The Winter Soldier, a yellow Penske truck makes two appearances that are crucial to the plot.

The truck first appears when Nick Fury is given a chance to escape when it crashes into his attackers. The second appearance is when Agent Sitwell is hit by it after being thrown into traffic. When asked if this identical truck model appearing twice was just a coincidence or an intentional choice, directors Joe and Anthony Russo confirmed that this was indeed the same truck.

When asked about the identity of the driver, the Russos joked that the driver might be up for his own Marvel one-shot, saying, “The man who drives that truck is very highly trained. He thinks on his own terms. He’s got a plan and a very specific skill set.”

While some critics wrote this off as the directors toying with overeager fans, others began speculating that this description fits the antihero character of the Punisher perfectly. Fans will have to wait and see if this tormented vigilante will make another appearance in the film, or if his brief undercover cameo in The Winter Soldier will remain a mystery.

The tragic backstory of Mantis

Mantis CGI Antennae - Guardians of the Galaxy 2

Little is explained about the backstory of Mantis when she first appears in Guardians of the Galaxy Vol. 2. All we really know is that she comes from a race of insectoid beings who have strong abilities of empathy.

However, the many different versions of Mantis' origin story that exist out in the comics are much more fleshed out, and none of them are particularly happy for Mantis herself.

In the most popular version, Mantis is raised and trained by Kree priests until her mind is wiped and she must work as a prostitute in Vietnam before eventually joining the Avengers.

Other versions of Mantis’s backstory involve her exploding and having the fragments of herself killed off one by one. Although these backstories may not be detailed specifically in the Guardians film universe, knowing her alternative pasts still brings a darker cloud to her bright character that is presented in the movie.

Paul Bettany didn’t work in-person with Robert Downey Jr. until Age of Ultron

Paul Bettany as Max Waters in Transcendence

If you aren’t familiar with the technical process of film, you might be surprised to learn that Paul Bettany, the voice of Iron Man’s A.I. assistant Jarvis, had not worked in-person with fellow cast members, including Robert Downey Jr. until Avengers: Age of Ultron. Even then, he only worked on set with Downey Jr. because he had been given another role in the franchise as Vision.

When asked about the film, Bettany said, “It’s really lovely to finally be working with these people that I’ve been working with for 10 years already.”

Up until that point, Bettany had only come into contact with the other actors during meetings or for press, since he only needed to show up for a few hours to record his lines at a studio without interacting with the other cast. “Now I have to show up and stuff,” he said.

The disturbing backstory of Rocket Raccoon

Rocket blasting through an enemy in Guardians 2

In Guardians of the Galaxy, Rocket Raccoon briefly alludes to his tragic past after a drunk fight in a bar with Drax. Although he doesn’t go into great detail, he mentions that only he knows what it’s like to be torn apart and put back together against his will, which we can assume took place over the span of multiple years.

This alludes to some sort of painful and inhumane experimentation, which is described in much more detail in the Marvel comics.

According to this origin story, Rocket Raccoon was originally engineered by cruel scientists not just for the purpose of providing scientific data through harsh experiments, but to keep insane patients of a mental asylum company, along with other animal friends. Eventually Rocket escaped the asylum and began an alternate career of crime and debauchery.

The true identity of Taserface

Guardians of the Galaxy 2 Taserface Chris Sullivan

When Taserface first appears in Guardians of the Galaxy Vol 2. he can pretty quickly be written off as another comic villain who exists to fit the wild, humorous adventures of the Guardians.

With a silly name and a narcissistic personality that rivals Gaston in Beauty and the Beast, Taserface doesn’t seem like much of an actual threat or sinister character. Knowing this, it might be surprising to find out that the film’s Taserface is actually based off a very real character in the comic books with a disturbing origin story.

Taserface in the comics faces off with the original Guardians in the year 3000, and can be directly connected with another character from the MCU-- Tony Stark.

Taserface belongs to the Stark race, which is a group of aliens that have constructed their society and way of life from Tony Stark’s litter, leftovers, and trashed technology that he shot into space thousands of years earlier. In other words, the cruelty of Taserface is an eventual result of Tony Stark’s casual disregard for littering.

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Can you think of any other Marvel Cinematic Universe secrets? Sound off in the comment section!