Kelp Forests

Kelp forests (synonyms: seaweed, kelp beds; example toponyms: The Seaweed Forests, The Seawoods) are subaquatic locations dominated by massive growths of kelp, a type of seaweed. Kelp forests occur primarily in arctic and temperate oceans on Earth near the coast. They may be near coral reefs or sea caves. In fictional worlds they may hide underwater ruins, underwater civilizations, underwater habitations, or shipwrecks. There may be aquatic predators or leviathans lurking in their depths, or they may be cozy areas depending on the world tone. Kelp forests may be intentionally cultivated, and may be a kelp farm.
When designing kelp forests for fictional worlds, consider the density of the kelp. There may be avenues or natural tunnels through the tangles. In fantasy worlds or science fiction worlds, kelp equivalent fictional plants may have unusual colors or properties.
Kelp is rich in iodine and alkali and kelp ash can be used for soap and glass production. Kelp also contains alginate, a natural thickening agent used in products like ice cream, jelly, and toothpaste. In addition, kelp is a food and can be eaten raw or cooked. All these qualities mean that it is often used as a game resource in game worlds with an aquatic element or as a trade resource in water worlds.

Who, or what, inhabits the kelp forests of your world?
Kelp is not a plant, it is a heterokont. Some species can grow as tall as 45 meters / 150 feet.