What's the biggest animal in the world by length, Blue Whale? Actually, it's a jellyfish called the Lion's Mane Jellyfish! One of these jellyfish were found to be 120 feet long, while the longest Blue Whale was nearly 111 feet. As we know, jellyfish sting, and the most deadly is the Australian Box Jellyfish. This jelly can kill from 20-100 people per year. One time while scuba diving, I got stung by a jellyfish at 60 feet deep. My arm burned intensely and I got disoriented and dizzy, with no clue how to communicate what was happening. I just pushed through until I could surface. Getting stung is no joke! What's the biggest animal in the world by length, Blue Whale? Actually, it's a jellyfish called the Lion's Mane Jellyfish! One of these jellyfish were found to be 120 feet long, while the longest Blue Whale was nearly 111 feet. As we know, jellyfish sting, and the most deadly is the Australian Box Jellyfish. This jelly can kill from 20-100 people per year. One time while scuba diving, I got stung by a jellyfish at 60 feet deep. My arm burned intensely and I got disoriented and dizzy, with no clue how to communicate what was happening. I just pushed through until I could surface. Getting stung is no joke!