The Kelp Forest is a place seen in the episodes Club SpongeBob and possibly Bummer Vacation, The Camping Episode and The Thing. It is a forest of many tall, green kelp that people frequently get lost in.
In the episode Club SpongeBob, SpongeBob's treehouse is flung into the air and lands in the Kelp Forest, getting SpongeBob, Patrick and Squidward lost in it. A forester finds them and asks the Magic Conch Shell what to do and it says to do nothing and they all do so, so it is unknown what happens to them. Also earlier in the episode a couple is seen lost in the Kelp Forest.
In the episode Bummer Vacation, Mr. Krabs drops SpongeBob off into a forest and throws his spatula into the forest and drives off when SpongeBob is not looking. It is unknown if this is the Kelp Forest because the kelp are more tree-like and in different colors and SpongeBob got out of the Forest so easily.
In the "Camping Episode" a SpongeBob and Patrick get lost in a forest similiar to the Kelp forest within Squidward's thought bubble.
In the episode "The Thing", another look-alike of the Kelp Forest is seen. It is seen when Smelly (Squidward) is taken back into the wild.