Jellyfish blooms are making headlines around the world. This is due to the damage they cause to tourism, fishing and our health. How can these creatures, which are over 98% water and have no shell, skeleton or brain, expand so rapidly? Although this is a normal stage in the life cycle of these gelatinous animals, we have to admit that blooms have become much more frequent and massive in recent decades. We're even witnessing jellyfish populations appearing in more and more regions where fish have been replaced by them, such as off the coast of Namibia, in the Black Sea, in the Sea of Japan and in certain areas of the Baltic Sea. What causes this? Why has the role of jellyfish in the ocean been underestimated, even though they outlived the dinosaurs? Are jellyfish on their way to dominating the oceans as they once did? What if they were to be the only ones left?