Please click to read the full article at Popular Science. "The black jellyfish (Chrysaora achlyos, also called the black sea nettle) is huge, with a bell (that's the dome-shaped part of the body) that can reach three feet across. But it's the rest of the body that's so scarily big: its arms, described by the Monterey Aquarium as "lacy and pinkish" can reach 20 feet long, and its tentacles can by nearly 8 feet long. It was only officially named and described in 1997, though it can be seen in photographs as far back as 1926. (It's clear in photographs, since it's the only dark-colored jellyfish in that part of the Pacific Ocean.)"