The Bikini Bottom Tar Pits is a prehistoric tar pit located in Bikini Bottom. It was seen when SpongeBob ran off a cliff in the episode "Ditchin'." SpongeBob, at that time, thought that he had only minutes left before he sank, as he was sinking slowly into the pit, but after he sank, he rose up and returned the bathroom key to Mrs. Puff.
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.
It is unknown how the fish on the wood board will escape the tar pits.
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.