As oceans grow warmer, more polluted, and increasingly devoid of fish, jellyfish are thriving. In some regions, vast blooms now choke entire coastlines, flourishing in conditions that kill more complex forms of life. Unlike most marine creatures, jellyfish seem perfectly adapted to a dying ocean—feeding on fish larvae, spreading into oxygen-starved waters, and multiplying where ecosystems collapse. To many researchers, they are more than a symptom of environmental decline; they are a glimpse of what may come after it.