Your mom's bikini bottom is home to Springboob, Patback, Mr. Kraps, and a whole lot more. Like the name implies, it is the bottom part of your maternal parent's swimsuit, and if she learned to wash that thing, she wouldn't have to deal with tiny creatures all up in there all the time. It makes for good TV, though.
Your mom's bikini bottom was first invaded by Fishy the Fish a million years ago. (Yes, she's that old. Deal with it.) There wasn't much to do back then besides sitting around and drinking soda pop. There were no bowling alleys, or stores, or newspapers, or theme parks, or places from The Simpsons that are in Spongebob for no reason. Fortunately, the future came along and invented all these things.