To the untrained eye, they're gelatinous blobs, drifting aimlessly on the ocean's currents. But jellyfish are among the most-feared and least-understood creatures on Earth. And their populations are exploding across the world. They lack sharp claws, piercing teeth or even a brain, but they are armed with an amazing arsenal of weapons. For hundreds of millions of years, jellyfish have been evolving, developing into the ultimate survivor. They have permeated every ocean on the globe, thriving in the arctic and the tropics. In an ever-changing world where other species struggle to endure, jellyfish populations are on the rise.