Over the last two decades, the proliferation of jellyfish has become a significant concern for our oceans. In New Zealand, divers grapple with millions of tiny, stinging jellyfish. These are no larger than grains of pepper. Meanwhile, in Sweden, an enormous cluster of moon jellyfish led to the shutdown of one of the world’s largest nuclear reactors by blocking essential pipes. In the Sea of Japan, massive Nomura’s jellyfish, weighing up to 200 kilograms and boasting bells two meters in diameter, swarm fishermen, wrecking nets and consuming local fish. Globally, these creatures voraciously consume fish eggs and larvae, thwart marine farming efforts, and outcompete adult fish by depriving them of essential resources. If things continue this way, we might face a future where the entire ocean is teeming with jellyfish.