The Bikini Bottom Tar Pits is a prehistoric tar pit that's located in Bikini Bottom. It only appears in the episode "Ditchin'."
There are no known inhabitants of the prehistoric pit, but there are fossils and bone structures that reveal that creatures from prehistoric times were there such as: dinosaurs, mammoths, etc.
Role in series
While escaping the Bikini Bottom Hug Festival, SpongeBob SquarePants falls off a cliff and lands into the Bikini Bottom Tar Pits. SpongeBob, at that time, thinks that he only minutes left before he sank for good, as he was sinking slowly into the pit. While sinking, SpongeBob cherishes those last kind words that Mrs. Puff said to him. After SpongeBob sinks all the way in the tar, he quickly rises up and realizes that he has to return a bathroom key to Mrs. Puff. SpongeBob gets out of the tar pits by pulling onto a prehistoric animal's tusk and climbing up a cliff.
The mammoth skull may be a confusion, as they never lived underwater, even though this may be a sea creature that looks like a mammoth.
Another possibility is that the tar pit was tall enough to reach the surface, where a mammoth was stuck in it, but over time the tar pit decreased height until it became the Bikini Bottom Tar Pits.
This place is a parody of the Tar Pits in North America, where extinct skeletons are found.