Bikini Bottom is the setting of the television show SpongeBob Squarepants. In the show, it is located underwater near a tiny island with a palm tree on it.
In GTA San Andreas, it is claimed to be located under a small island in the middle of the ocean. Under the water would be a vast world consisting of all the known locations in the television show, including the Krusty Krab, SpongeBob's house, and many others. There are also Boatmobiles used for transportation there, instead of normal cars.
With a huge lack of evidence supporting the myth, it is widely known as false, due to copyrighted names that are used, and the fact that there is a mod for it.
It is also notable that San Andreas is set in 1992, while Spongebob Squarepants first premiered in 1999.