The largest jelly - known as the Lion's Mane jellyfish - grows an average 8 feet in diameter. The largest on record had tentacles 120 feet long! Its sting, however, will cause a human only minor irritation. There are a couple of Jellyfish that do not sting, including the strange Cannonball Jellyfish.
Here's an interesting fact - Most jellies do not have brains, but the Box jellyfish actually has four brains!
But what really draws me to them is that most are absolutely beautiful!
See the 17 Most Beautiful Jellyfish Species on Earth
The Atolla jellyfish is on this list. And it's not only beautiful, it has a unique defense mechanism. When threatened it does an amazing thing. See this incredible video of the Atolla lighting up, presumably to attract larger predators to kill its attacker.