It looks much like a normal drain, but larger. It also has a brown chain attached to it to pull the drain if necessary. If pulled out, it will drain the entire ocean.
Role in episode
This drain serves as the main plot point of the episode. Mr. Krabs explains to SpongeBob and Patrick of how dangerous it is to pull drains. Then he describes the main drain and retells a story about how two children were wandering around and pulled the drain for fun.
SpongeBob and Patrick decide to set out and find the drain, but later realize that Mr. Krabs and Plankton were the two kids that pulled the drain. Then, Patrick pulls the drain, causing it to suck all of Bikini Bottom into it. However, everyone realizes it was just a story. SpongeBob was just reading a story to Patrick the whole time.