In the deep sea, life had to evolve to accommodate an alien-like world: There's little to no light, it's extremely cold and the pressure is immense. For years, scientists assumed very few organisms could live in an ecosystem so extreme, but we now know that's not the case. Many organisms evolved to produce their own bioluminescent light or evolved special organs to lure food, which is more scarce than in the upper ocean. Among other adaptations, some fish species have hinged jaws to accommodate much larger meals and some are flattened to reduce their risk of predation.
To check out some of the strange and fantastic ways creatures under the sea have evolved, here is a video capturing 25 of the most terrifying-looking sea creatures.