Superpowers, almost by their very definition, are impossible. People can't shoot lasers out of their eyes or grow to be the size of skyscrapers without breaking every rule in the science textbook. Whether they defy the laws of physics, biology, chemistry, or all three, superpowers are conceivable, but not realistic. However, there are some comic book characters with abilities that are at the very least inspired by real-world phenomena. In an industry where most writers don't offer any logical explanations for their characters' abilities, there are a handful of writers that attempt to apply (somewhat) logical explanations for characters' superpowers. And sometimes, powers that were once thought to be impossible have been discovered and/or replicated in real life. Of course, given that these are comic books, even the most scientifically coherent superpowers are pushed past the natural into the supernatural. Still, these are the characters with superpowers with some grounded explanations.
The world of comic books is meant to be a place of fantasy and science fiction. Occasionally though, a character will possess abilities that mirror the natural world (even if they ultimately exaggerate it).
Of course, the idea of a mass of plants that transforms into a humanoid creature with the memories of a dead scientist is far-fetched. But the connection that DC's Swamp Thing, who is such a creature, has to the Green has some basis on real-world plant behavior. The Green is an elemental and sentient hive mind that connects all plant life on the planet. Plants aren't sentient - in the sense that a human is sentient - but they display a kind of sentience in their behavior. They can communicate over vast distances through intricate underground systems of roots and fungi. For example, suppose a tree is attacked by caterpillars. In that case, the tree will send messages through pheromones and electrical signals to its neighbors to alert them of the danger of infestation so that they may increase their defenses. Trees also use this system to share resources with others who are malnourished or sick. These systems can stretch for miles and connect numerous plants. In the comics, Swamp Thing's connection to the Green is treated as magical, but it actually can reflect the kind of connection that plants have with one another.
Carin Taylor, aka Velocity, has the power of superspeed and is a member of the Image Comics superhero team CyberForce. While there are plenty of speedsters in comics like Flash and Quicksilver, Velocity is one of the few that directly addresses the complications such a power would bring and doesn't brush it off with something like DC's Speed Force. Carin was given cybernetic enhancements by the evil corporation Cyberdata. With these enhancements, she can run at speeds up to 3300 mph. To prevent friction burns from rapid movement, Cyberdata implanted layers of Kevlar on Carin's skin for protection. Computer chips were also injected into her brain, enabling her to process information faster and to react more quickly, which is crucial for her rapid movements. Although not all the kinds of impediments of achieving such impossibly swift speeds are explained, Velocity's origin, at least, tries to address many of the issues real-life speedsters would face.
While there was a time Sue Storm's power of invisibility seemed like something only possible in comic books, science has shown that an invisibility power is possible. This may be achieved by using materials that bend light waves around an object so that a viewer will see what is behind the object instead of the object itself. Total invisibility hasn't yet been achieved, but partial transparency has been made possible and with some special cloaks. Sue's power works just like these real-life invisibility cloaks, as she can bend light around herself or objects by manipulating photons. This illusion makes them imperceptible to the naked eye.
After being exposed to toxic chemicals, Matt Murdock, aka Daredevil, lost his sight but his other senses were enhanced to a superhuman level. While many blind people learn to enhance their other four senses to make up for their lack of sight, Matt's abilities are beyond anything a normal human can do. The closest real-life comparisons to his abilities are echolocation and blindsight. Echolocation involves making sounds and listening to the echoes produced when the sound bounces off nearby objects. Blindsight, on the other hand, is a neuropsychological phenomenon where a blind person still can react to visual stimuli, even though they can't consciously see them. Matt's abilities are an exaggerated version of these two phenomena. Still, they do show that there is some penumbra of truth in Daredevil's super-senses.
One of the grossest mutants in Marvel also has one of the most realistic superpowers. Alex Cluney, aka Zeitgeist, has the power to vomit an incredibly corrosive and deadly acid. However, this corrosive effect isn't that far off from the capabilities of actual human stomach acid. To digest your food, the acid in your stomach is so strong that it can break down skin, bones, and even iron. The inner lining of your digestive track protects you from this acid. In real life, the little acid you do vomit up isn't as dangerous because it's not concentrated. Also, of course, stomach acid doesn't work nearly as fast as what is depicted in the comics, which dissolves objects in seconds. Even so, Zeitgeist's power is just an enhanced version of a very real part of the human body.
The X-Men are mutants born with superpowers ranging from teleportation to rewriting reality itself. However, Forge's mutant power is just being an "intuitive genius." Forge is very smart and instinctively knows everything about technology. In a world with people like Iron Man and Hank Pym, can that even be called a superpower? Many people in the real world have demonstrated similar unconscious skills across various fields, from art to mathematics. Through years of training and studying, people can develop the ability to intuitively and instinctively complete complex tasks. Many of history's most significant figures, like Mozart and Albert Einstein, showed similar expertise and skill in their fields, and they didn't need an X-gene to do it.
Kick-Ass is an Image Comics series about portraying "realistic" superheroes. The inability to feel pain, the superpower of the titular protagonist, Dave Lizewski, reflects an actual rare medical condition. Kick Ass is a nerd who dreamed of becoming a superhero. He donned a green costume to fight crime and was immediately stabbed and hit by a car. After that disastrous first outing, the doctors had to put metal braces and plates inside his body. This had the side effect of making Dave lose 85% of his body's pain receptors. With this ability, he can fight harder and stronger without being held back by pain. The inability to feel pain is an actual rare genetic condition known as Congenital Insensitivity to Pain (CIP) or Congenital Analgesia. Thus, Kick Ass's power represents a very real condition. However, it's far from a beneficial superpower because it means that any external or internal damage they receive is masked and can go unnoticed and untreated, leading to debilitating injuries.