Tonga

There once was a young film director who wanted to make a name for himself. Not through his stories, for he mostly made live action remakes of beloved animated movies, but through the use of his wealth. He wanted to do something improbable, nay near impossible. He wanted to break the record for the deepest dive a human has ever made.
But before he could do that he had to test his submarine, so he visited the Great Barrier Reef, the bottom of the icebergs of Antarctica, the wreck of the OceanGate, and the West Mata volcano, and now for its final test: the Tonga Trench. He hired a crew to sail out to the southwestern pacific Ocean. And when the weather was mild and the waves were calm he had them lower his yellow submarine into the sea. He had it painted that way so it would make a more iconic picture on his social media.
Deeper and deeper he went, until he reached the bottom of the Trench. There he saw fish in all kinds of aberrant shapes and sizes. Some with translucent bellies, others with spikes in all kinds of places. He saw thousands of sea stars feeding off whale carcasses, and a lost piece of the Apollo 13. He thought he had seen everything there was to see, when suddenly he spotted a wide tunnel, leading deeper into the subduction zone. He could film this zone, which had given the great minds of yesteryear the proof they needed for the theory of plate tectonics, solving the mystery of deep sea quakes. Down the tunnel he went.
He went down winding tunnels and shallow caves, going ever forward into unknown depths. The waters around him became warmer and warmer, so whispered the dial of his thermometer. The caves were becoming thinner and thinner, to the point where a single wrong move would get his submarine stuck forever. Then he suddenly found an large cavern containing a bubble of air. He wasn't sure if this air was breathable, but curiosity drew him out of his submarine to take a look with his own two eyes. Here in this cavern saw wonders no man had ever seen before. He saw crystals made of pure rare earth elements, rocks from which he could squish water with his bare hands, and small arthropods that hadn't seen light since the first tetrapods crawled onto land.
Had he not entered the last cavern, he might have gone on to become rich and famous beyond his wildest dreams. But alas he did, following the smell of something rotten, something ancient, something that every fiber in his body told him not to follow. It smell of a thousand dredged riverbeds, whose fish population had long ago starved and rotted because of a swarm of algae on its surface. He entered the cave, and when he shone his flashlight on the source of the smell his mind broke. He was face to face with an eldritch being, with bulbous unblinking eyes and needle-like teeth, sitting on a pile of rock as soft as pudding. From its chin hung a long fleshy string with a bioluminescent tip, its serpentine body was covered in several swollen translucent stomachs, and its four light gray pectoral fins hanging like great wings alongside its body. The last coherent thing his crew on the surface heard from him was the director stammering “D...D...Dragon!” into his radio, followed by a string of laughter, before giving way to eternal silence.