When it comes to the Marvel Universe, there might not be a more powerful superhero than The Incredible Hulk. He has been around almost since the start of Marvel Comics and has fought just about everyone -- hero and villain -- and rarely loses a fight. Even when he does fall to defeat, it is normally his own fault. Not only is Hulk "the strongest there is" but he is also someone whose rage can increase his strength and power and make him almost unbeatable, invulnerable and possibly immortal. With that said, on top of his massive strength, Hulk has some interesting facts about his actual skin that makes him one of the more interesting characters in comics.

When Bruce Banner first became the Hulk, he was originally supposed to be gray. However, the color was changed to green, which is the form most fans know Hulk as now. However, outside of the color of his body, Hulk's skin has a number of qualities that help him not only stand out among other heroes but also survive where others would perish. Add in other Hulks -- like his cousin She-Hulk or his nemesis Red Hulk -- and these heroes have some of the most fascinating skin traits of any comic book character!

15 STAN LEE WANTED HIM TO BE GRAY

When Stan Lee created The Hulk in 1962, the big green goliath was not supposed to be green at all. Originally, Lee wanted Hulk to be gray because he believed it was better to have him as a non-color so as not to connect to any human race or living being. In Stan Lee's book, The Origins of Marvel Comics, he said that he loved the Frankenstein monster, who he did not believe was a bad guy, as well as Jekyll and Hyde, and wanted to make Hulk a cross between those characters -- which have also been portrayed as shades of gray -- which Lee also considered a "spooky" color.

The problem is that there was a printing error when the first issue of The Incredible Hulk arrived. The gray shade that Lee asked for would not print right and it looked either silver or charcoal black. As a result, Lee chose to change Hulk to green since there was no other major character in Marvel Comics at that time who was green. It also helped Lee create the nickname "The Green Goliath." However, that did not end the consideration that Hulk could be gray, and it was an idea that Marvel returned to over 20 years later.

14 HULK'S POWERS WHEN GRAY

The Hulk returned to his gray color scheme in 1986 with Incredible Hulk #324, a comic book that actually recreated the cover of the first Hulk issue, but with the gray Hulk. The gray Hulk then changed his name to Joe Fixit and set up shop in Las Vegas where he was a morally ambiguous enforcer and was much less heroic than the monster Hulk even was. The entire idea when Hulk's skin is gray is that he is cagey, manipulative and doesn't care about anyone other than himself.

Hulk also isn't as strong when he has gray skin, although he can still lift about 75 tons. The entire adrenaline rage that powers up Hulk still works here but it takes a lot more to get Hulk mad enough to Hulk-out. This is mostly because gray Hulk doesn't have the savage rage problems as his green counterpart, which is why his strength and power levels are lower when his skin is this color. There was also originally a different reason for turning into Hulk. When his skin was gray, Bruce Banner would turn into Hulk at night and turn back at dawn instead of changing due to stress. Another fun difference for a gray-skinned Hulk is that he can drive cars.

13 HULK'S POWERS WHEN GREEN

When Hulk is green-skinned, he has different powers than he did when he was gray. Unlike the gray Hulk, who turns that color when the sun went down, green Hulk changes when he gets stressed out and/or injured. This is the Hulk that almost everyone knows about -- the one who gets mad and then stronger as he grows more agitated, angry or injured. He eventually succumbs to Savage Hulk if the pain grows unbearable. This Hulk is pretty much unbeatable, indestructible and invulnerable to anything -- with very few exceptions.

This version of Hulk was introduced in The Incredible Hulk #1 and he has been green-skinned longer than he has been anything else over his existence. Hulk is not always dumb when he is green, and there are times that he can talk and other times where he is actually intelligent, but most of the time when his skin is green, Hulk is just someone who acts on emotions and normally fights villains or those he feels presents a threat. This Hulk is also the biggest threat to property when he is involved in fighting anyone. Most alternate versions of Hulk -- such as Amadeus Cho and Maestro -- maintain the green skin.

12 HULK WAS ALSO ONCE "PALE GREEN"

When Hulk is green, most of the time he is simple and runs on emotion and aggression. However, there was a period of time after he became Mr. Fixit and had gray skin that Hulk kept his intelligence and maintained almost complete control of his emotions -- all while retaining the powers of green-skinned Hulk and the morality that was lost when his skin was gray. That Hulk was a mix of the two and most people refer to him as the "pale green"-skinned Hulk, or the more widely known as a sort of later iteration of the "Professor Hulk."

So, the idea here was slightly ridiculous. After Hulk was gray and remained morally ambiguous in his role in Las Vegas, he ended up going to see a therapist. That doctor was able to bring together the three personas of Hulk (green, gray, and Bruce Banner) and turn them into one being -- the pale-green Hulk. This version wore flip-flops and developed a defensive mechanism where he reverted to Banner in case he got too angry so he wouldn't lose control and Hulk out. When Hulk's skin was this shade, he was not as powerful as the basic green Hulk because he was in control of his emotions but was able to become a true hero again by shaking the immorality of his Mr. Fixit persona.

11 SHE-HULK IS THE OPPOSITE OF HULK WHEN GRAY

When The Incredible Hulk was gray, he was smart and devious and morally ambiguous. He could talk and wear suits, even if he was a bit smaller and weaker. When Hulk was green, he was emotional and had times he would lose control to unquenchable rage episodes. However, when it came to his cousin Jennifer Walters, the She-Hulk was the exact opposite of Hulk. When She-Hulk is green, she wears regular clothes and even operates as an attorney while in her green state.  But when She-Hulk turns grey, she becomes savage.

In Sensational She-Hulk #15, Jennifer was surrounded by demons and ended up Hulking out more than she ever had before. The angrier she became, her skin turned from green to grey and she began to lose intelligence, starting to talk like her cousin The Hulk and smashing everything in her path. The story showed that when rage consumed She-Hulk, the color of her skin changes as she begins to lose control of everything. She-Hulk again turned gray after her battle with Thanos in Civil War II and the loss of her cousin Bruce Banner, once again allowing the stress to cause her to lose control of what makes her civil and intelligent.

10 ONLY TWO THINGS CAN CUT HULK'S SKIN

When someone attacks Hulk with a knife, a sword, bullets or just about anything else man-made, it will bounce off Hulk's skin or break completely. There is a reason Hulk can withstand anything thrown at him. His powers include immense durability thanks to several hundred pounds of fortified bone and muscle that can withstand almost any attack. However, as far as Earth-based weapons, there are two things that have pierced Hulk's skin in Marvel Comics -- Adamantium and Vibranium.

Of course, Adamantium is what Wolverine uses and he has used them to great effect in Savage Wolverine when he stabbed Hulk in the brain to stop him from destroying a machine that could unleash a monster upon the Earth. Vibranium can also cut the Hulk due to the same reasoning as Adamantium -- they are two of the strongest metals in the world. The one thing to note is that when Hulk gets angrier, his skin becomes more durable and can then withstand even these metals. In some interesting trivia, the Ultimate Universe added a failsafe for Hulk when they showed that Vibranium turns brittle when exposed to gamma radiation (Ultimate Avengers 2), and since Hulk is always letting off levels of gamma rays, it isn't the same in that world.

9 HULK'S SKIN CAN WITHSTAND SOME MAGIC

When it comes to magic attacks, the durability of Hulk's skin is all dependent on the story and what the writers want at the time. However, when it comes to the basic gist, even Hulk's skin is invulnerable to magic swords. An example of this came in Incredible Hulk #207 when Hulk fought Valkyrie. In the books, Valkyrie has a sword called Dragonfang that is enchanted and almost indestructible. In that issue of Incredible Hulk, Valkyrie used it against Hulk and it could not pierce his skin

However, things change in other places. For example, in Thor: Ragnarok, Hulk was teaming up with the movie version of Valkyrie, Thor and others to battle Hela and her demon wolf Fenris. In Norse legend, it is Fenris that kills Odin during the coming of Ragnarok, so the beast is extremely powerful. In the movie, Fenris and Hulk battle on the Bifrost bridge and when Fenris bites Hulk, it draws blood -- something that very few creatures -- mythological or otherwise -- have ever been capable of achieving. However, Fenris is an exception to the rule, and Hulk's skin is normally impervious to almost any physical attack -- whether from a god or a mortal.

8 HULK CAN RESIST UP TO 1 MILLION DEGREES OF FLAME

Hulk is also able to resist great flame, and he has done it many times before. The biggest examples of Hulk resisting fire came when he went one-on-one with the Human Torch. Johnny Storm can control heat energy and create fire, which he can engulf himself in or even shoot at someone else. The normal temperature of the Human Torch's flame is 780 degrees Fahrenheit, and no harm comes to Johnny. Of course, great harm can come to anyone or anything that he comes in contact with. This brings us to the Nova Flame.

Whenever the Human Torch is burning at his highest and hottest levels, he can go "nova." When he takes all his body's energy that is stored away and then focuses it into one spot for a nova burst, it reaches up to one million degrees Fahrenheit.  This is the level of a nuclear warhead detonation and can destroy an area of about 900 feet in diameter. A perfect example was in World War Hulk #2 when Torch realizes that nothing is stopping Hulk and blasts him with his most powerful shot, an attack that ends with Hulk standing unfazed and unharmed while Johnny lies on the ground unconscious.

7 HULK'S SKIN HAS REGENERATIVE POWERS

As the comics have shown, there are times that Hulk's skin is damaged; when that happens another power of the Hulk kicks in. While Hulk can suffer damage (such as with Adamantium or Fenris' bite), he will bleed and feel discomfort. However, much like Wolverine and Deadpool, Hulk has a regenerative healing factor and it is just as strong as theirs, even though few people talk about this specific power.

If Hulk's skin is punctured by Wolverine's claws, the holes will quickly close up and heal and any damage to internal organs will also regenerate and heal. This means that Hulk's skin is not only completely impervious to damage but when it does take damage, it almost immediately and instantaneously heals itself. In Incredible Hulk #398, Vector extracted almost all his skin and muscle mass and it regenerated with the skin and muscle growing back in seconds. Also, just as Hulk gets stronger and more invulnerable when he grows angrier, getting really mad also increases the speed of his healing powers, which is what makes Hulk almost completely unbeatable in a battle. The Incredible Hulk can also repel poison and chemical substances the angrier he becomes.

6 SHE-HULK'S HEALING POWERS ARE WEAKER

Much like her cousin, She-Hulk also has regenerative powers and can patch up almost any hole quickly to get back into action. When Jennifer Walkers turned into She-Hulk, she adds on 560 pounds of muscle and bone tissue, but it is more condensed, so she looks leaner than Hulk. She-Hulk is still able to withstand bullets and many weapons, but swords and blades can cut her easier than they do the Hulk. While Hulk can withstand Human Torch's greatest blasts, Jennifer can only withstand up to 3,000 degrees Fahrenheit. She can withstand many other things, though, like cannon shells and energy blasts.

Much like her durability, She-Hulk also has a slightly less ridiculous regenerative power. One reason might be that she is in complete control of her emotions and doesn't get as big of a power boost that Hulk does when he rages. When it comes to the healing factor, She-Hulk will rapidly regenerate injuries in just minutes. However, unlike Hulk, who can grow back limbs and internal organs, She-Hulk does not have that power and can't grow back anything that is cut off or destroyed completely. She can, however, heal tissue damage and is immune to drugs, toxins and diseases.

5 HULK'S SKIN CAN WITHSTAND LACK OF OXYGEN

When most heroes go into outer space or into the depths of the ocean, they have to wear a special suit. However, Hulk does not need one. Unlike someone like Namor, whose body is made for deep underwater living, or Thor, who can fly in outer space without worries, Hulk survives in both of these areas for a different reason. Hulk's skin has the regenerative powers that allow it to instantly heal himself. Hulk's skin can resist freezing in maximum cold or blistering in maximum heat. It regenerates itself when deep water or space exploration would cause a normal being's skin to break down.

This stretches to breathing in space and underwater as well. In Hulk #77, Hulk realized that his lungs filled with pressure and he was suddenly able to breathe underwater as his body worked hard to morph to heal itself. Even Nick Fury said in World War Hulk that he could survive in outer space without oxygen due to his body's mutations, simply saying that it is proven in their data on the hero. As World War Hulk also showed before he attacked Black Bolt on the moon, Hulk is not really bound by lack of gravity either, and his skin is completely protected when flying through space.

4 HULK'S SKIN RADIATES GAMMA RAYS

One of the most interesting things about Hulk's skin and something that is rarely mentioned is that it is always radiating gamma rays. This was described in Incredible Hulk #103 as Hulk's body being an actual "gamma battery." This means that the gamma explosion that turned Bruce Banner into the Hulk flooded him with so much gamma radiation that his body is always regenerating it. Hulk's body is constantly creating more gamma radiation and radiating it from his skin.

In Incredible Hulk #464, Hulk was battling Armageddon and proved that he could control the rate that his body exerted the gamma radiation, flooding a machine and causing it to explode to beat the villain. Interestingly, Hulk can also use the gamma rays coming out of his skin to repel other forms of energy, which he did when fighting The Avengers in Incredible Hulk #128. Hulk can also use his body to absorb more gamma radiation when he needs to. He actually did this once when fighting Red Hulk, using his body to absorb Red Hulk's gamma energy to drain him and beat him in a fight. It has never been determined how much control Hulk has on the gamma rays emerging from his skin.

3 RED HULK GETS STRONGER WHEN SKIN IS EXPOSED TO RADIATION

Red Hulk has one clear advantage over The Incredible Hulk. When Red Hulk is exposed to radiation of any form, especially gamma radiation, he grows exponentially stronger. Yes, Hulk can use his "gamma battery" within to drain Red Hulk of his gamma energy, but he has to have the opportunity to do so or Red Hulk has an advantage. See, Hulk is always emitting gamma radiation from his body, and when Red Hulk gets close, he absorbs that radiation and becomes stronger.

Red Hulk is nowhere near as strong as The Incredible Hulk, but his ability to absorb energy helps even the playing field somewhat when the two do battle. According to the Official Handbook of the Marvel Universe, Red Hulk is at a strength level of 100 without absorbing energy, compared to Hulk's unlimited strength. However, when Red Hulk absorbs energy (including cosmic energy), that number jumps exponentially and he can absorb it at will, allowing his cunning and military training to allow him to use it as a weapon on demand. Red Hulk is never able to reach the levels of power of an enraged Hulk, but he is on the level with someone like Thor or Hercules when he absorbs energy into his skin.

2 RED HULK'S SKIN HEATS UP AND BURNS OUT IN LONG FIGHTS

One of the Red Hulk's biggest weaknesses is the fact that the longer he fights, the hotter his skin becomes, and he soon burns out -- literally. The angrier that Red Hulk becomes, the hotter his skin gets as well. He leaves large amounts of radiation behind when he gets this angry or physically exhausted. The Incredible Hulk has unlimited stamina and can fight for extended periods of time without tiring. However, Red Hulk has proven in Thunderbolts #31 that, when he gets too hot and exerts too much energy, he changes back to General Ross and he is out of the fight until he can regain his energy levels and cool down.

The heat that Red Hulk's skin lets off is also dangerous to anyone around him, making him a liability when working in a team. When he grows angrier and his skin gets hotter, it can turn sand into glass and eventually cause Red Hulk to grow weaker (Hulk vol. 2 #6). The reason for this is that, while Hulk can rage out and grow stronger due to a power boost, Red Hulk's body cannot deal with the excess heat (Hulk vol. 2 #23), which makes him a liability as well when the fight goes on. As such, Red Hulk will never win an extended battle against someone like Hulk as a result.

1 HULK'S BODY MIGHT BE A GIANT BRUISE

Everything on this list is based on what has happened in the comic books, what was explained by the writers, or what Marvel has published in their guides about the Hulk. However, there are some people who love comic books so much that they pour attention and research into every detail. When scientists do this for superhero comics, it can make for fascinating reading. A biologist at the University of Stanford did this when it comes to both Captain America and The Incredible Hulk.

A postdoctoral research fellow in biology at Stanford named Sebastian Alvarado looked at both the stories and origins of Captain America and Hulk and tried to determine the real science behind the heroes. For Cap, Alvarado said that there are genome-editing tools that could possibly create a super soldier. As for Hulk, the biologist explained that gamma rays could pull switches that could transform someone if it was a high enough level. In skin bruising, red blood cells die, the hemoglobin breaks up, and the blood looks green after that, which is why bruises are discolored. This would explain the color of The Incredible Hulk's skin -- which Alvarado said could be a full body bruise due to the body trauma.