A growing body of evidence suggests the planet is in the midst of a sixth mass extinction.
Between 500,000 and 1 million plant and animal species face extinction, many within decades, according to a report from the United Nations. Pollution, habitat loss, warming oceans, and other consequences of climate change are driving animal populations down on an unprecedented scale.
But one group of creatures is bucking this ominous trend: jellyfish.
Jellyfish have roamed Earth's oceans for 500 million years. The bell-shaped underwater denizens can be found all over the world; there are some 4,000 species of them, according to the Smithsonian Institute.
Over the past two decades, global populations of many jellyfish species have skyrocketed. Swarms of them, known as "jellyfish blooms," have become more common worldwide, forcing beach closures, causing power outages, and killing other fish.
Recent research has revealed that the increases in jellyfish populations can be linked to human activity, too. As greenhouse gases trap heat on the planet, oceans are heating up — they absorb 93% of that excess heat. Unlike many marine species, jellies can thrive in warmer water with less oxygen.
What's more, their natural predators, like turtles and sharks, are being overfished by humans.
Here's what to know about why jellyfish are thriving — and why their population explosion could be dangerous.