It looks much like a normal drain, but larger. It also has a brown chain attached to it to pull the drain plug 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 plugs. Then he describes the main drain and retells a story about how two children were wandering around and pulled the drain plug for fun.
SpongeBob and Patrick decide to set out and find the drain, but later find out that Mr. Krabs and Plankton were the two kids that pulled the drain. Then, Patrick pulls the drain plug, causing it to suck all of Bikini Bottom into it. However, it is revealed that it was just a story. SpongeBob was reading a story to Patrick the whole time.