Many of the worldbuilding details in The Legend of Korra plunged headfirst into the mythology of the Avatar world. Characters would talk openly about the Spirit World or engaging monologues about their personal history, and at one point, Korra even spent a two-episode arc focused on the first Avatar and the origin of bending.
But a super subtle detail may have been hiding in plain sight in the midst of the show without you even realizing it. In fact, if it were a shark, it may have bit you.
Sandbending was one of the more mysterious bending arts in the original series. It did not even come up during the first season of the show, and when it finally did in the second, it was only in a scant and thinly detailed manner. The sandbenders of the Si Wong Desert capably used earthbending to navigate their environment, creating small tornados within their sand-sailers to soar across the dunes of their home. It even proved a plot point that the earthbending prodigy Toph could not pick the skill up so easily.
The Blind Toph "saw" through the use of her seismic sense, which picked up vibrations in the ground around her, and in the Si Wong Desert, everything appeared blurry and fuzzy, throwing off her aim when she tried to attack the sandbenders and buzzard wasps that plagued the landscape. Although later Toph did manage to gain mastery over sand, it only came with practice. It would appear that the badger moles that first taught Toph earthbending did not include a lesson on shaping sand.
Therein lies a peculiarity with the history of bending in the world of Avatar so far unaddressed: how did the sandbenders of the Si Wong Desert first pick up the skill? Toph's later development of metalbending and the opening of Toph's Metalbending Academy proved that most new forms of bending become simpler once they are pioneered in the first place, but if the badgermoles who first taught humans to earthbend could not sandbend, who first developed the technique?
The Legend of Korra presents an interesting possibility: the sand sharks. Seen in one episode during Book Three while Korra and Asami are stranded in the Si Wong Desert, an enormous sand shark hunts them down, swimming through the subterranean sands as easily as its aquatic counterparts do through the ocean. The gargantuan beast is so jaw-dropping in its scope and power that it's easy to overlook one important detail: what it's doing shouldn't be possible without bending.
No beast is able to swim through sand as easily as water, and if one as large as the sand shark did, it would massively alter the landscape in its wake. There would be giant sinkholes or dunes created by the sand shark's path, and that's assuming it could even burrow and surface as simply as it does in the first place. The way it glides through the desert is almost magical, and that's the most intriguing possibility of all.
If sand sharks were the original sandbenders then it would be a valuable piece of Avatar lore fleshing out the backstory of a people seldom seen and even more rarely detailed. Badgermoles aren't the only bending animals: sky bison and dragons are seen doing the same, but at no point is it ever stated that those three animals are the only animals in the world of Avatar who can bend the elements.
The original series may get much of the credit, but The Legend of Korra was certainly capable of subtle world-building all on its own. If something like the sand shark doesn't seem to present much mythology at first then you may just want to dive a little deeper.
