Kelp is a type of large brown seaweed that grows in cool ocean waters, forming vast underwater forests along rocky coastlines. These aren't the small green strands you might see washed up on a beach: kelp can grow over 100 feet tall, reaching from the seafloor toward the sunlight at the surface. Some species grow as much as two feet per day, making kelp one of the fastest-growing organisms on Earth.

Kelp forests create entire ecosystems beneath the waves. Fish hide among the swaying fronds, sea otters wrap themselves in kelp to keep from drifting while they sleep, and sea urchins graze on the kelp itself. Without sea otters to eat the urchins, the urchins can devour so much kelp that whole forests disappear.

People harvest kelp for food and other uses. If you've eaten miso soup or certain Japanese dishes, you've probably tasted kelp, which is rich in vitamins and minerals. Kelp is also processed into a thickening ingredient called algin that’s used in ice cream, toothpaste, and even some science experiments. Next time you see brown seaweed on the beach, you might be looking at a piece of an underwater forest that supports thousands of ocean creatures.