The locker, to anyone viewing it from the outside, looks like a normal, metal locker, with hot pink-colored velvet carpeting that SpongeBob put in. On the inside, however, it is very large. It was converted into a filing room, so all the walls have drawers each with its own code, in a number. It is written in number-letter form (for example, Squidward's clarinet was originally placed in drawer 36.8-B, then supposedly moved to shelf 1018.2-E, but in actuality, it was 2019.3-F).
Almost nothing is known about the locker, except that Mr. Krabs used it when he was in the Navy. When Corporal Sterling, a Naval officer, died, his bones were put in the locker as a prank. It had not been used until Mr. Krabs brought it out. It was very dirty and still had Sterling's skeletal remains in it. According to Mr. Krabs, the locker had not been cleaned since the sixth boot camp.
Role in episode
Squidward demands a place to put his clarinet after it is drooled on by a baby. Mr. Krabs gives him the locker, but makes him share it with SpongeBob. SpongeBob puts his disgusting toothbrush in the locker, but keeps adding more and more things, so he creates an enormous filing room out of it, with multiple stories, and thousands of drawers containing his personal items and Squidward's clarinet.
When SpongeBob shows Squidward the locker for the first time, it is just one huge room, but when Squidward goes inside later after hearing a lot of construction noises, he finds that it has been expanded a whole lot more, and now consists of many floors and many more drawers. Squidward chases SpongeBob throughout the locker, and eventually finds himself in the Horned Forest.
On the bottom row of one wall on the second floor, there is a small crawlspace where a drawer should be. On the other side of the crawlspace is a what appears to be a storage space behind a stage, with a large purple curtain. The Horned Forest is on the other side of the curtain.