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The dragons on "Game of Thrones" aren't so cute anymore. In season one, they looked like pet lizards with wings. During the fifth season, which starts Sunday on HBO, they grow into much larger, dangerous creatures.
Obviously, these are mythical creatures. George R.R. Martin, the author of the "A Song of Ice and Fire" books that "Game of Thrones" is based on, has already attributed their abilities to magic. But what if some mad scientist tried to create a dragon? And where did this idea of fire-breathing dragons come from in the first place?
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In "Game of Thrones," there are currently three dragons: Drogon, Rhaegal and Viserion. To avoid any spoilers, let's just say at this point in the books, they are kind of like growing, rambunctious teenagers. The largest dragon in "Game of Thrones" mythology was Balerion, the Black Dread, which could swallow an entire mammoth.
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One estimate put Balerion at 61 meters (200 feet), which is longer than most jetliners. Overall, the dragons in "Game of Thrones" are not that different from the dragons in the "The Hobbit" and other fantasy novels, except they are often bigger and ridden like giant, terrifying horses by members of the royal family.
In "Game of Thrones," dragons swoop down from the sky, reigning terror on people below. In real life, they would have trouble walking, let alone flying.
"These dragons look like a scaled-up version of a modern reptile, like a lizard or crocodile," Michael Habib, a paleontologist at the University of Southern California and an expert on the flight mechanics of pterosaurs, told NBC News.
Legs that are close to the ground can only support so much weight, which is why giant dinosaurs like Brontosaurus had long, straight legs.
"With crouched lizard limbs, you have to have proportionally thicker bone because you are bending and twisting them a lot," he said. All that stress would crack the bone in a creature as large as a dragon. To walk around Westeros, beasts like Balerion would need to stand more upright, like an elephant.
Flying would be out of the question. Quetzalcoatlus, the largest pterosaur, was 14-feet tall and had a 35-foot wingspan. That's big, but not the size of a jetliner. If something that gigantic tried to fly, its wings would probably break, Habib said.
Fantasy authors did get one thing right: dragons would probably have bat-like wings instead of the wings of a bird, because they can support more weight. But the bones needed to support the flexible membrane of a dragon wing would need to be truly massive.
"In 'Game of Thrones,' the dragons would have so much bone supporting their wings that there wouldn't be enough membrane left over to allow them to work as wings," he said.
Also, thick scales might protect from arrows, but they also add weight without providing any extra muscle, a big no-no when it comes to flying.
"It's not a coincidence that we don't have heavily armored animals flying around," he said.
Except for circus performers, there aren't any living beings that blow fire. There is, however, a creature that sprays a boiling, toxic chemical brew from its body.
There are more than 1,000 species of the bombardier beetle worldwide. This feisty bug stores hydrogen peroxide and chemicals called hydroquinones in a reservoir located in its abdomen. When it feels threatened, it empties that mix into a reaction chamber, where enzymes trigger a chemical reaction.
In some beetles, according to University of Arizona entomologist Wendy Moore, that mix can reach 100 degrees Celsius (212 degrees Fahrenheit) before the hot, smelly, toxic substance is ejected onto potential predators.
"That reaction chamber is very hard and very small," Moore told NBC News. "It's kind of like the chamber is armored, and it protects the rest of the body from the reaction."
Bombardier beetles are expert marksmen with the ability to aim in nearly every direction. Might an angry dragon be able to utilize something similar to shoot a fireball at its enemy?
"I've never really thought about it in relation to a dragon," she said, laughing.
Sure, 212 degrees isn't hot enough to melt steel and bring down the walls of Harrenhal. It's more appropriate for cooking a nice stew in a crock pot. Still, if a mad scientist were creating a dragon, a thick reaction chamber where chemicals could be heated would not be a bad choice.
Unlike the White Walkers, dragons existed in human myths thousand of years before "Game of Thrones." Early on, they were often depicted as serpents, like the dragon that guarded the Golden Fleece from Jason and the Argonauts.
It's not entirely clear where that image of a dragon came from. One theory is that boa constrictors — now an exotic pet — were once a deadly threat to human beings.
"We don't think of them as dangerous today, because we don't sleep in beds of reeds with doors that don't close," Matt Kaplan, author of "The Science of Monsters" and the upcoming "Science of the Magical: From the Holy Grail to Love Potions to Superpowers," told NBC News.
Back when men wandered the wild without machetes or guns, they were a lot more vulnerable to snake attacks. (Even now serpents will occasionally kill people). It's not hard to imagine that these snakes inspired early dragon tales, although the stories might also have been stoked by crocodiles or fossilized dinosaur eggs, or possibly a combination of all of those factors.
But what about the classic Western dragons that breath fire and fly and look more like lizards than snakes? They first appeared in Northern Europe. Between 500 to 600 A.D., a tale emerged about a king named Vortigern who hid in his castle in Wales from angry Saxons. Unfortunately for him, the castle walls kept falling.
In the story, a young boy tells the king that the cause of his falling walls are dragons, who are fighting each other with fire in underground caverns. That boy was Merlin, the famed wizard of Arthurian legend. The reality was more likely connected to rich coal deposits near the castle site in Wales, according to Kaplan.
"If you go digging underground, especially with metal instruments, you are going to start sparks and you're going to start fires," he said.
Later, around 700 to 800 A.D., a fire-breathing creature also appeared in the legend of Beowulf. In Scandinavia at that time, important people were buried with their servants and prized animals. Those bodies would have created a build-up of flammable methane gas.
When grave-robbers opened the tomb, metal tools and torches in-hand, they could have sparked the flames that inspired dragons in Scandinavian poetry. That would have also reinforced the idea that dragons guarded treasure.
A professor named J.R.R. Tolkien fell in love with the tale of Beowulf, wrote his own translation, and then created the flying dragon Smaug for his 1937 novel "The Hobbit."
"It's a classic Western vision of a dragon," Kaplan said. "Tolkien really solidified the idea of what a dragon should be, and 'Game of Thrones' uses that Tolkien-esque vision of a dragon."
So there you go, "Game of Thrones" fans—the mighty Balerion was inspired by by Scandinavian grave-robbers who were scared by exploding methane gas.